Abstract:
A method for booting a processing device, the processing device comprising a first and a second processing unit, the method comprising: detecting by the first processing unit, whether at least one boot configuration parameter is accessible from a non-volatile storage medium of the processing device, the at least one configuration parameter being indicative of a boot interface; if said at least one configuration parameter is available, forwarding at least a part of the detected at least one configuration parameter by the first processing unit to the second processing unit; otherwise detecting by at least one of the first and second processing units whether a boot interface is available to the processing device; booting at least the second processing unit from the indicated or detected boot interface.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/743,444, filed Sep. 3, 2006, the disclosure of which is fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to the booting of processing devices comprising a first and a second processing unit. 
     BACKGROUND 
     One example of a processing device comprising a first and a second processing unit includes a mobile platform, i.e. a chipset/integrated circuit for use in a plurality of different mobile communications devices. A mobile platform can be used in several different hardware configurations including e.g. a mobile phone architecture using two central processing units (CPUs). In such a two-CPU mobile phone architecture typically one of the CPUs is used as an access CPU that handles the communication/real-time constrained tasks, and the other CPU is used as an application CPU that handles the phone application tasks. It is a cost advantage to include both the application and the access CPU on the same baseband digital application specific integrated circuit (ASIC). However, in order to allow the platform to be used together with more powerful application systems, it is generally desirable to be able to use the platform together with an external CPU and application system instead of the application CPU of the platform. For example, an electronic device including the platform may be connected to another data processing system such as a computer via a suitable interface, e.g. via a universal serial bus (USB). Such a configuration in which an external CPU is used instead of the internal application CPU of the platform is also called a bridge configuration. In this case, there is no direct use of the internal application CPU. 
     According to a first aspect, It is thus desirable to provide an initialization or boot process for the processing device that facilitates both situations in an efficient and cost-effective way. 
     European patent application EP 1 486 869 discloses a boot process for initializing a co-processor of a system including a main processor and a co-processor. Even though this process avoids the need for a NOR flash memory to be associated with the coprocessor it still requires two or more flash memories connected to the respective processors. 
     According to another aspect, the booting of a processing device such as a mobile platform for normal operation typically requires that certain basic or platform software, e.g. an operating system and/or firmware, and possibly certain configuration parameters have been installed on the processing device, e.g. during manufacturing of the device or a subsequent configuration. This installation is typically performed by loading the software onto the processing device, e.g. into a non-volatile memory such as a flash memory of the device. To this end the processing device can typically be operated in a special mode of operation, referred to as software flashing mode or service mode, in which the processing device is adapted to load software over an external interface so that the device can be configured for normal use. The process of loading the basic software and configuration parameters will also be referred to as external load. 
     In some mobile platforms external load is indicated by a service pin that is connected to the access CPU. The service pin can for example be trigged when the user enters a specific keyboard combination. Once the service mode is detected, the platform loads the software to be executed from an external interface, instead from the internal non-volatile memory, e.g. flash memory. However, according to the second aspect, it remains a problem to provide a boot process that facilitates operation in a service mode irrespective of the hardware configuration. 
     SUMMARY 
     According to the first aspect, disclosed is a method for booting a processing device, the processing device comprising a first and a second processing unit, the method comprising:
         detecting by the first processing unit, whether at least one boot configuration parameter is accessible from a non-volatile storage medium of the processing device, the at least one configuration parameter being indicative of a boot interface;   if said at least one configuration parameter is available, forwarding at least a part of the detected at least one configuration parameter by the first processing unit to the second processing unit; otherwise detecting by at least one of the first and second processing units whether a boot interface is available to the processing device;   booting at least the second processing unit from the indicated or detected boot interface.       

     Consequently, the above boot process may be performed independently of whether the processing device boots in a normal configuration, i.e. using both its processing units or in a bridge configuration in which only one of the processing units is used, thereby providing a general-purpose start-up or boot procedure for the multi-processor device. 
     In particular, embodiments of the boot process described herein do not require the presence of a flash memory, and may thus be used in processing devices operated in different hardware configurations. 
     Furthermore, embodiments of the boot process described herein do not require a pure hardware-implicit bridge configuration, i.e. an entirely hardware-based detection of a bridge configuration based on which interfaces are connected, since bridge interfaces such as USB may also be used also for other non-bridge purposes. 
     For a manufacturer of a mobile platform it is an interesting advantage to be able to produce a general purpose platform including a single boot program that can boot irrespective of the specific hardware and software configuration it may be chosen to be operated in. For example, it is an advantage of the boot process described herein that it allows provision of a low cost mobile platform for use in smart phones or in modem products such as USB plugs etc., where the platform is bootable even without any large non-volatile memory like a flash memory. 
     The detected or indicated boot interface may be an internal interface, i.e. an interface to another module/unit included in the processing device, or an external interface, i.e. an interface for connecting to an external device. Examples of an internal interface include an interface to a non-volatile memory included in the processing device. Accordingly, the external CPU is external to the chip/chipset/integrated circuit board of the mobile platform. The external CPU may be a CPU in the same processing device, e.g. a CPU on a separate integrated circuit board, or it may be a CPU of a separate device different from the processing device that includes the mobile platform. 
     In one embodiment detecting whether one or more boot configuration parameters are accessible from a non-volatile storage medium of the processing device includes detecting whether the processing device includes a non-volatile memory for storing configuration parameters, and if the processing device includes a non-volatile memory for storing configuration parameters, detecting whether the detected non-volatile memory has stored thereon a data file including the one or more configuration parameters. Examples of configuration parameters may include security parameters such as software version information, a customer ID, platform hardware configuration parameters, such as a bridge/non-bridge flag, a bridge interface identification, and/or the like. 
     Since the boot polling order of the boot procedure initially attempts to find bridge configuration information in the non-volatile platform storage when such memory is available and the platform is suitably customized, the boot procedure described herein works particularly efficiently in configurations with non-volatile storage on the mobile platform system. This is advantageous, since such configurations are typically used for mass market products with stringent start-up performance requirements. Nevertheless, since in the absence of stored bridge configuration information, the process polls possible external interfaces to detect whether any bridge configuration information is available from any of these interfaces, the boot process can also be performed in other “flash-less” configurations. 
     In one embodiment booting at least the second processing unit from the indicated or detected boot interface includes receiving boot software from the identified or detected boot interface, i.e. software for performing at least a part of the boot process. When receiving the boot software further comprises performing a security check of the boot software by at least one of the first and second processing units before execution of the received boot software, an increased security is provided against attempts to boot the system with unauthorised software or by an unauthorised user. For example, the security check may include a verification of the authenticity and/or the integrity of the boot software and/or the authenticity and/or authorisation of the provider of the boot software, or the like. The security check may include a cryptographic verification process, e.g. a private and/or public key based cryptographic verification process. 
     In one embodiment, performing the security check is performed by one of the first and second processing units functioning as a security root for software verification during booting. In one embodiment, the method comprises reading, by the processing unit functioning as a security root security information, wherein the security information is stored protected, e.g. cryptographically protected, in a non-volatile storage medium of the system. Consequently, the most security sensitive functions are confined to one of the processing units, thereby further reducing the risk of malicious attacks. 
     In one embodiment the method comprises performing, by the first processing unit, a sequence of protocol interactions of a predetermined boot sequence, where only a subset of the protocol interactions is conditioned on said detection whether the one or more configuration parameters are available. Examples of protocol interactions include the exchange of messages, requests, responses, etc., with the second processing unit and/or a storage medium and/or external interfaces. In one embodiment the subset includes less than 5 interactions. Accordingly, when the boot process is constructed such that in the different configurations the respective sequences of interactions only differ from each other in one or a few interactions, a compact boot software may be provided that is applicable irrespective of the hardware configuration. Hence, the boot processes, even though generic, does not require large amounts of memory in the device, and is cost-effective to maintain and install. 
     According to the second aspect, disclosed is a method for booting a processing device, the processing device comprising a first and a second processing unit, the processing device being selectably bootable in one of a stand-alone configuration and a bridge configuration; wherein, in the stand-alone configuration, the first and the second processing units are initialised to be operational, and wherein, in the bridge configuration, only the second processing unit is initialised to be operational and initialised to be in operational connection with an external processing unit via a communications interface; the method comprising:
         detecting whether the processing device is to be booted in the stand-alone or in the bridge configuration;   if the processing device is to be booted in the bridge configuration, receiving a boot mode indication from the external processing unit via the communications interface, the boot mode indication being indicative of whether the processing device is to be booted in a service mode, in which the processing device is configured to load software from the external processing unit into a non-volatile memory of the processing device;   responsive to the received boot mode indication booting the processing device in said service mode.       

     Hence, it is an advantage of embodiments of the boot process described herein that it allows booting a platform device both for normal operation and in a service mode, irrespective of whether the device is operated in a stand-alone configuration or in a bridge configuration. 
     For example, in a mobile platform USB bridge solution, i.e. a mobile platform that uses USB as communications interface between the mobile platform access CPU and an external CPU system, the boot process described herein allows the external system to indicate whether to boot the platform in a service boot mode or a normal boot mode, without requiring a service pin or other hardware configuration, since an USB connection typically would not provide a connection of the service pin to the external system. Embodiments of the boot process described herein thus provide a generic boot procedure also for “flashless” bridge configurations and configurations without service pin service indication. Nevertheless embodiments of the process may facilitate that the service mode may be indicated by a hardware configuration such as a by setting a pin connected to one of the CPUs or by a protocol interaction with an external system. 
     It is a further advantage of the boot process described herein that it provides a generic boot process that can work efficiently, e.g. without unnecessary start-up delays, even for non-bridge and/or flash configurations. 
     In one embodiment, the method further comprises receiving, if the processing device is to be booted in the stand-alone mode, a boot mode indication via a user-interface of the processing device, the boot mode indication being indicative of whether the processing device is to be booted in the service mode. Consequently, the boot process also allows for an indication of a service mode by a user via a user interface of the device. For example, this indication may be provided by a service pin of one of the processing units. 
     In some embodiments, the processing device is a communications device for providing at least one communications service, wherein the first processing unit is an application central processing unit adapted to execute at least one application software component for providing functionality different from the communications service, and wherein the second processing unit is a communications central processing unit adapted to control the communications service. For example, the processing device may be a platform circuit for one or more mobile communications products, wherein the at least one communications service includes a cellular telecommunications service. Nevertheless, it will be appreciated that the method may also be applied to other types of processing devices. 
     In one embodiment, each of the first and second processing units includes a corresponding read-only-memory having stored thereon boot code for controlling at least a part of the booting of the corresponding processing unit. Hence, the boot procedure is controlled at least in part by ROM-based code on both processing units. In addition to the boot code stored in the ROM, the boot process is controlled at least in part by boot software stored in writable memory of the device or loaded from an external system via a bridge interface. Hence, at least a part of the boot software may be altered, thereby facilitating maintenance of the device. 
     When the method comprises communicating boot information between the first and second processing units by means of a predetermined boot protocol, an efficient boot procedure for a multi-CPU architecture, e.g. a 2-CPU architecture is provided. 
     It is noted that the features of the methods described above and in the following may be implemented in software and carried out on a data processing device or other processing means caused by the execution of program code means such as computer-executable instructions. Here and in the following, the term processing means comprises any circuit and/or device suitably adapted to perform the above functions. In particular, the above term comprises general- or special-purpose programmable microprocessors, Digital Signal Processors (DSP), Application Specific Integrated Circuits (ASIC), Programmable Logic Arrays (PLA), Field Programmable Gate Arrays (FPGA), special purpose electronic circuits, etc., or a combination thereof. 
     For example, the program code means may be loaded in a memory, such as a RAM (Random Access Memory), from a storage medium, such as a read-only memory (ROM) or other non-volatile memory, such as flash memory, or from another device via a suitable data interface, the described features may be implemented by hardwired circuitry instead of software or in combination with software. 
     The present invention relates to different aspects including the method described above and in the following, corresponding devices, and computer programs, each yielding one or more of the benefits and advantages described in connection with the above-mentioned methods, and each having one or more embodiments corresponding to the embodiments described in connection with the above-mentioned methods. 
     In particular, according to one aspect, a processing device comprising a first and a second processing unit is suitably configured to perform the steps of the method described above and in the following. 
     For the purpose of the present description, the terms processing device and electronic device comprise any portable radio communications equipment and other handheld or portable devices and/or components such as integrated circuit boards thereof. The term portable radio communications equipment includes all equipment such as mobile telephones, pagers, communicators, i.e. electronic organisers, smart phones, personal digital assistants (PDAs), handheld computers, media players, such as mp3 players, digital cameras or other recording devices, embedded devices in the automotive industry, medical devices, or the like. 
     According to another aspect, a computer program product comprises computer-executable instructions adapted to cause, when executed on a processing device comprising a first and a second processing unit, the processing device to perform the method described above and in the following. In some embodiments, the computer program product is embodied as a computer-readable medium, such as a read-only-memory or a re-writable non-volatile memory, having stored thereon the computer-executable instructions. 
     For the purpose of the present description, the terms storage means/device and computer-readable medium are intended to comprise any suitable storage medium, device or circuit, e.g. a read-only-memory (ROM), a random access memory (RAM), a flash memory, an Erasable Programmable Read-Only Memory (EPROM), volatile or non-volatile memory, an optical storage device, a magnetic storage device, a diskette, a CD, a hard disk, or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects will be apparent and elucidated from the embodiments described in the following with reference to the drawing in which: 
         FIG. 1  shows a schematic block diagram of a mobile platform including two CPUs. 
         FIG. 2  shows a schematic block diagram of a mobile platform including two CPUs in a bridge configuration. 
         FIG. 3  shows a schematic block diagram of a mobile platform including two CPUs in a non-bridge configuration. 
         FIGS. 4   a - e  show a flow diagram of an example of a boot process for a mobile platform. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic block diagram of a mobile platform system including two CPUs. 
     The mobile platform system, generally designated  100 , includes two subsystems: an access subsystem  101  and an application subsystem  102 . The access subsystem includes an access CPU  110  while the application subsystem  102  includes an application CPU  120 . For example, the mobile platform system  100  may be an integrated circuit/chipset for use in a mobile terminal or other communications equipment. The 2-CPU architecture of the mobile platform system  100  thus facilitates a functional split between the access subsystem and the application subsystem. For example, the access subsystem  101  may be configured to handle one or more standardised communications protocols and/or other functionality that require real-time control in which meeting deadlines in a timely fashion is important. The application subsystem  102  on the other hand may be configured to handle end-user functionality and/or other functionality not requiring real-time control. 
     Various interfaces may be part of the application subsystem and the access subsystem, respectively. For example, the application interface may provide one or more interfaces such as, a display interface  121 , a camera interface  122 , an audio (e.g. microphone and/or loudspeaker) interface  199 , and/or further interfaces (not explicitly shown) such as a keyboard interface, a smart card interface, a memory stick interface, and/or the like. The application subsystem is further shown to include a graphics accelerator  181 . 
     Similarly, the access subsystem  101  may include communications circuitry  112 , e.g. GSM a GSM/GPRS module  161 , a GSM cipher block  162 , a GPRS cipher block  163 , a WCDMA module  164 , and a WCDMA cipher block  165 , a digital signal processor (DSP)  166 , and/or the like, and provide one or more further communications interfaces  182 , such as an infrared data association (IrDA) interface, an universal serial bus (USB) interface, a Bluetooth interface, a universal asynchronous receiver/transmitter (UART) interface, a serial peripheral interface (SPI), an inter-integrated circuit interface (I2C), and/or the like. The access subsystem further include a One-Time-Programmable memory (OTP)  167 , e.g. for storing a chip-unique identifier and/or other parameters. The access subsystem may further provide an interface  168  to a Universal Integrated Circuit Card (UICC), such as a SIM card, a USIM card, or the like. 
     The access subsystem may further include security modules, such as a platform integrity module  169  for providing platform code and data integrity checks, a crypto accelerator block  170  for providing efficient computation of cryptographic values, such as key generation, message authentication, etc., a random number generator  171  for use in e.g. key generation, and/or the like. 
     Each of the access subsystem  101  and the application subsystem  102  includes a ROM  103  and  104 , respectively, each including corresponding boot code  105  and  106 , respectively. The boot code in the respective ROMs is adapted to perform at least an initial part of the boot process, e.g. the boot process until the boot software from the internal memory or the external system is loaded. Furthermore, the boot code stored in ROM  103  of the access subsystem  101  provides the platform security root functionality. In a mobile terminal with an access subsystem and an application subsystem where the application subsystem may be disabled when configured in a bridge configuration, it is an advantage that the access subsystem functions as a security root, since the access subsystem is always available regardless of the chosen configuration. 
     Each of the access subsystem  101  and the application subsystem  102  further includes Instruction and Data Tightly Coupled Memories (ITCM/DTCM)  173  and  174 , respectively. The ITCM is on-chip memory into which an initial part of the boot code is loaded. Furthermore, each of the access subsystem  101  and the application subsystem  102  is shown with a service pin  183  and  184 , respectively. In each subsystem, the respective components are interconnected via at least one suitable bus  185  and  186 , respectively, e.g. a high speed bus or a high speed bus and a peripheral bus, and/or the like. The access subsystem and the application subsystem communicate with each other via a suitable interface  113 , such as a communications interface between the access and application CPU, e.g. a serial link, one or more shared memories, and/or the like. 
     The mobile platform system  100  may include one or more memory controllers for controlling access to one or more internal memories. In the example of  FIG. 1 , the mobile platform includes a memory controller  105  for controlling a common random access memory (RAM)  150  shared by the access and application subsystems. Hence, the memory controller  105  functions as memory arbiter which is configured by the access subsystem. For example, the memory controller may be configured such that respective memory regions are access-protected from the application system, i.e. the controller can prevent access from the application system to certain memory regions that belong to the access system. Alternatively or additionally, the platform system may include separate RAMs for the respective subsystems. Similarly, the mobile platform includes a static memory controller  106  for controlling one or more non-volatile memories, e.g. a flash memory such as NAND flash memory and/or a NOR flash memory, and a corresponding static memory controller  106 , in  FIG. 1  shown connected to the application subsystem. For example, during operation of the mobile platform system in a stand-alone configuration, software for the access subsystem  101  and the application subsystem  102  may be loaded from a flash memory connected to static memory controller  106  to the RAM  150 . For the purpose of the present description, it will be assumed that the memory/memories is/are accessed from the application CPU. However, as will be discussed below, the boot procedure described herein is also applicable for a mobile platform system that does not include non-volatile memory. The application subsystem of  FIG. 1  is further shown to include a further memory controller  180 . 
     The access subsystem and the application subsystem may be implemented on the same chip or as separate chip sets interconnected via a suitable interface. While the access and application CPUs are always present, in some configurations a further, external CPU may be connected to the system as described herein. It will further be understood that alternative implementations of a mobile platform system may include additional and/or alternative components. Examples of such mobile platform systems are disclosed in international patent application WO 2005/041601. 
     As will be described in greater detail below, the boot code is either loaded from an external interface, i.e. in the bridge or service mode case, or from flash memory included in the platform system, e.g. flash memory attached to e.g. interface  106  or  180  in  FIG. 1 . Irrespectively of whether the boot code is loaded from an external or an internal interface, the initial boot code to be loaded is stored on the ITCMs of the respective CPUs. 
       FIG. 2  shows a schematic block diagram of the mobile platform  100  described in connection with  FIG. 1  in a bridge configuration. The mobile platform system  100  is shown connected to an external system  230 , including an external CPU  231 , via one of the interfaces  182  of the platform system, e.g. via a USB interface. The external system  230  in  FIG. 2  is further shown to include a graphics accelerator module  280  providing a camera interface  222  and a display interface  221 , as well as a memory controller  205  for controlling access to a RAM  250  and a flash memory  251 . The external system of  FIG. 2  further includes an audio interface  299 . The external system  230  may also be used to boot the mobile platform in service mode so as to perform an external load. It will be appreciated that the external system may include alternative and/or additional components. 
     In the system of  FIG. 2 , the mobile platform itself does not include a flash memory and may thus be referred to as a flash-less bridge configuration in which the mobile platform system is booted via the bridge interface  182 . In the configuration of  FIG. 2 , the application CPU  120  may be disabled during the boot process as described herein, since during normal operation and after completion of the boot process, the external CPU  231  performs the functions of the application system. 
       FIG. 3  shows a schematic block diagram of the mobile platform of  FIG. 1  when configured in a non-bridge configuration. The mobile platform  100  is identical to the platform shown in  FIG. 1 , but includes an additional RAM  350  and a flash memory  351 . The flash memory  351  may thus include boot configuration information and/or security configuration information for use by the mobile platform during the boot process. When the contents of the flash memory are integrity protected, the security of the system is increased. In some embodiments, all the memories included in or connected to the mobile platform system (such as the RAM  150 , the RAM  350 , the flash memory  351 , the ITCM/DTCM  173  and  174 ) are integrity protected. 
     In the following an embodiment of a boot procedure for a mobile platform system as described above will be described in greater detail with reference to  FIGS. 4   a - e . In one embodiment of the boot procedure which is applicable irrespective of the hardware configuration, the access CPU determines from which interface or memory to read the boot software and the application CPU determines or receives information from the access CPU whether to read the boot software from an interface or internal memory. For the purpose of the present description, it will be assumed that if the mobile platform includes a non-volatile memory this memory is accessible by the access CPU, thus providing a particularly low-complex solution. However, it will be appreciated that the method may be modified so as to cover implementations where the non-volatile memory is accessible via the application CPU. 
       FIGS. 4   a - e  show a flow diagram of an example of a boot process for a mobile platform. 
     The embodiment of a boot process for a 2-CPU mobile platform system shown in  FIG. 4  involves at least the access CPU  110  and the application CPU  120  of the mobile platform. The boot process may further involve an external CPU  231  when the mobile platform system is booted in a bridge configuration or another external computational entity  340 , such as a PC, functioning as a service device when the mobile platform system is booted in service mode. 
     In particular,  FIGS. 4   a - b  show the steps performed by the access CPU  110  and the PC  340 , while  FIGS. 4   c - d  show the steps performed by the application CPU  120  and the external CPU  231 , and  FIG. 4   e  shows an overview over the entire boot process. Horizontal lines indicate messages/signals communicated between the access CPU  110 , the application CPU  120 , the external CPU  231 , and the service device  340 , respectively. 
     The access and application CPUs  101  and  102  reside on the mobile platform, while the external CPU  231  is comprised in an external system, e.g. an external device, and external chip set, or the like, and connected to the platform system in a bridge configuration. When the platform system is not booted in a bridge configuration, the external CPU  231  is not present and mobile platform system does not receive any message from the external CPU. In case of a non-bridge configuration and when the mobile platform system is booted in a so-called “service mode”, the mobile platform system loads the boot software from another external entity  340 , e.g. a PC. When the system is not booted in service mode, but e.g. in a normal operation mode, the service device  340  is not present. 
     The boot process of  FIG. 4  is initiated by a reset signal  1  received by the access CPU  110  and the application CPU  120 , causing the access CPU at step  2   a  and the application CPU at step  2   b  to perform a platform reset, including power on, an initial synchronisation as indicated by synchronisation, and/or the like. If the mobile platform system is booted in a bridge configuration, the reset signal  1  may come from the external system  231  that typically will boot first. The trigger signal  1  may be forwarded to trigger further systems. Furthermore, the access CPU at step  2   a  and the application CPU at step  2   b  perform an initialization of hardware blocks and a potential initialisation and checking of the internal CPU communication with a handshake as indicated by horizontal lines  402  and  403 . 
     In some implementations the application CPU  120  may be configured to detect any bridge interface using hardware settings. For example, the application CPU may check the signals on some external pins. The signals may be configured to allow the application CPU to read configuration information. If this is the case, the application CPU detects the bridge interface at step  3 . Alternatively, the access CPU may perform this detection (this option is not shown in  FIG. 4 ). 
     In step  4 , the application CPU  120  detects whether bridge configuration information is available on non-volatile storage connected to the application CPU  120 . For example, in a configuration where the mobile platform system includes a flash memory, such configuration information may be stored in a configuration file of the flash memory. 
     If the application CPU  120  has detected a bridge interface in step  3  or found bridge information in step  4 , the application CPU forwards the relevant information to the access CPU  110  via message  5 . However, as mentioned above, in some configurations the mobile platform system may not include a non-volatile memory, or the memory may not include configuration information. In this case the application CPU will not be able to obtain this information in step  4 , and it may inform the access CPU via message  5  accordingly, thus causing the access CPU to initiate an interface polling sequence as described below. 
     In steps  6   a  and  6   b , the access CPU  110  and the application CPU  120  each read the service PIN status, i.e. detect whether a service pin connected to the respective CPUs is set. In steps  7   a  and  7   b , the access CPU  110  and the application CPU  120 , respectively, determine the mode of operation (service/non service), and proceed accordingly. If the service pin indicates service mode, the access CPU continues at step  8 , while the application CPU awaits a message/signal from the access CPU. Otherwise, i.e. if service mode is not detected, the access CPU proceeds at step  9  and the application CPU proceeds at step  10 . 
     In step  8 , i.e. if service mode was detected at step  6   a , the access CPU  110  checks whether any of the potential external service mode boot interfaces (USB, UART, etc.) are connected to an external system  340 . For example, the access CPU may check all its applicable interfaces in a predetermined polling order. If any interface is connected, this interface is selected. The entity connected to the interface may be an external CPU  231 , i.e. the mobile platform system may be booted in a bridge configuration in service mode, or the entity connected to the detected interface may be a different computational entity  340 . Accordingly, the access CPU proceeds at step  18  and determines whether it is the external CPU that is connected to the service interface. 
     In step  9  the access CPU  110  determines whether the access CPU  110  has received bridge configuration information in step  5 , i.e. information as to whether a bridge configuration applies and on which interface. If the access CPU has received information indicating a bridge interface, the access CPU continues at step  11  and switches to the detected bridge interface. Otherwise, the access CPU continues at step  12  and checks whether an external CPU is connected to one of the applicable external bridge boot interfaces of the access CPU (e.g. USB, UART, MSL, SPI, and/or the like). If an external CPU is connected to any of the interfaces, the access CPU selects the detected interface; otherwise the boot interface is determined to be an internal interface to a non-volatile memory of the mobile platform system. The access CPU sends a request  14  for operation mode information (e.g. service/normal) to the external CPU. If the access code does not detect any connected interface at step  12  the sequence is false and it goes for reset. 
     Similarly, the application CPU determines in step  10  whether a bridge configuration was detected in step  3  or step  4 . If a bridge configuration was detected, the application CPU awaits a configuration acknowledgement message  16  from the access CPU; otherwise, the application CPU awaits a message  25  from the access CPU as described below. 
     In case of a bridge configuration where an external CPU  231  is present, the external CPU  231  detects in step  13  whether the mobile platform system is to be booted in service mode. For example, the external CPU may receive a user command/input and initiate a boot of the mobile terminal platform in service mode in response to the user command/input. 
     Upon receipt of the request  14  for service mode information from the access CPU, the external CPU determines that the access CPU is ready for exchanging data. The external CPU then sends service/normal mode information  15  to the access CPU. If the access CPU fails to receive this information, the access CPU determines that an unknown configuration applies and aborts the boot process. 
     If the bridge configuration was detected by the application CPU, the access CPU sends, upon receipt of the service/normal mode information  15 , a configuration acknowledgement  16  to the application CPU confirming the service mode detection. In response to the configuration acknowledgement  16 , the application CPU sends a request  17  for service mode status to the access CPU. 
     After exchanging messages  14  and  15  and, if applicable, messages  16  and  17 , the access CPU continues at step  19 . 
     At step  18 , the access CPU determines whether the service interface detected in step  8  is the same as the bridge interface, i.e. whether the connected external interface detected in step  8  is connected to the external CPU in a bridge configuration. If this is the case, the access CPU proceeds by sending a service mode request  20   b  to the external CPU  231 ; otherwise the access CPU sends a service mode request to the service device  340 . It will be appreciated that the distinction between the requests  20   a  and  20   b  in  FIG. 4  is mainly for diagram consistency reasons, as the access CPU merely sends the service mode request to the service interface that was detected in step  8 . 
     At step  19 , the access CPU determines whether the mode indication  15  from the external CPU indicates operation in service mode. If this is the case, the access CPU continues by sending a service mode request  20   b  to the external CPU  231  as described above; otherwise, if a bridge configuration was detected, the access CPU sends a bridge configuration message  23  to the external CPU; otherwise, the access CPU directly sends a configuration acknowledgement message  25  to the application CPU. 
     Hence, the access CPU sends the service mode start request  20   a  or  20   b , respectively, to the respective external system, i.e. the service device  340  or the external CPU  231  when in bridge configuration. The service mode start request is a request for preparing the system for boot over the external interface. For the purpose of the example shown in  FIG. 4 , the service mode in bridge configuration is thus assumed to be performed via the bridge interface and not over any of the other possible interfaces of the access subsystem. However, it will be appreciated that the boot sequence may readily be extended to cover also the latter case, e.g. by performing step  8  at this stage, i.e. after the detection of the bridge interface. 
     In steps  21   a  and  21   b , respectively, the access CPU determines whether a cable or other connection was detected on any of the interfaces or whether a time-out occurred. If no cable was detected or if a time-out occurred, the access CPU aborts the service mode process and proceeds in normal mode instead, i.e. by sending the bridge configuration message  23  to the external CPU. 
     Otherwise, the external system, i.e. the service  340  or the external CPU  231  as the case may be, acknowledges the service mode start request via messages  22   a  or  22   b , respectively. Upon receipt of this acknowledgment, the access CPU continues by sending message  23  to the external CPU or message  25  to the application CPU, depending on whether a bridge configuration was detected or not. 
     If a bridge configuration has been detected, the access CPU  110  sends a bridge configuration message  23  to the external CPU  231 . The message  23  may also include information on whether a fallback to normal mode was done or not, e.g. in step  21   a, b , respectively. 
     Upon receipt of the bridge configuration message  23 , the external CPU  231  returns an acknowledgment  24  together with configuration information from the external CPU, such as flashless or flash configuration information, 
     Subsequently, the access CPU sends a message  25  to the application CPU  120 : If service mode request  17  was received by the access CPU, i.e. if a bridge configuration was detected, the message  25  includes a response with service mode information. In this case, the message  25  may include the service mode information that the access CPU received from the external CPU in message  15 . If no bridge configuration was detected, the message  25  includes a configuration acknowledgement message to the application CPU instead. 
     In response to message  25 , the application CPU  120  returns message  26  so as to signal that the application CPU is ready to start the security checks and then the software boot loading process. 
     In response to message  26 , the access CPU acknowledges in message  27  the ready to start. 
     The following steps of the boot sequence depend on whether the boot process is performed with non-volatile memory on the mobile platform system. Accordingly, in step  28  the external CPU  231  determines whether the external CPU  231  is configured for a boot with non-volatile storage on the mobile platform system. If this is the case the external CPU continues the boot process of its own system independently from the mobile platform system from this point until both systems are up and running. Otherwise the external CPU  231  awaits a message  39  from the access CPU indicating that the access CPU is ready for performing security checks. 
     Similarly, in step  29 , the access CPU determines whether a bridge interface has been detected (in step  12 ) but no non-volatile storage on the platform has been found, e.g. in the case of a flashless bridge configuration or in the case of an uncustomized flash. If this is the case, the access CPU switches to the bridge interface in step  38 . 
     Otherwise the access CPU proceeds at step  31 , where the access CPU  110  invokes a number of security checking routines. In one embodiment, the security checking routines include a check of the platform security configuration and a check of the software loading. 
     At subsequent step  33 , the access CPU reads security hardware settings, if applicable, i.e. if such security hardware settings are present as part of a given implementation of the mobile terminal platform. Examples of such security hardware settings include One-Time-Programmable Memory, e-fuse registers etc. These settings may be used to verify security configuration and software to be loaded. 
     Similarly, in response to the ready signal  24 , the application CPU  120  invokes in step  30  a corresponding number of security checking routines. In one embodiment, the security checking routines include a check of the platform security configuration and a check of the software loading. 
     In step  32 , the application CPU reads security configuration parameters from the non-volatile storage medium. If no memory or parameters are detected, the application CPU saves this state information in a suitable internal memory such as a RAM. 
     The application sends the security configuration information  34  obtained at step  32  to the access CPU. If the application CPU has not found any configuration, the application CPU informs the access CPU about this fact. 
     If there is no non-volatile memory on the platform (as determined in step  29 ), the access CPU may simply disregard the security configuration information received from the application CPU, as indicated by step  35 . 
     In step  36 , the access CPU checks the received security configuration information. For example the check may include an integrity check of the received security configuration. After successful completion of the security check, the access CPU returns an acknowledgment  37  of the receipt of the security configuration information to the application CPU, and the access CPU proceeds at step  47 . 
     In step  47 , the access CPU continues with the boot process from the detected (internal or external) interface. The boot process may include the downloading of software via the detected interface, the security (e.g. integrity) checking of the downloaded software, and execution of the software. 
     As described above, if the access CPU in step  29  has determined that a bridge interface has been detected (in step  12 ) but no non-volatile storage on the platform has been found, e.g. in the case of a flashless bridge configuration or in the case of an uncustomized flash, the access CPU switches to the detected bridge interface in step  38 . 
     Subsequently, the access CPU sends a signal  39  to the external CPU  231  via the bridge interface indicating that it is ready to start the security checks and then the software boot loading process. 
     The external CPU  231  returns an acknowledgement  40  of the ready to start signal  39 . 
     In subsequent step  41 , the access CPU invokes the security checking routines, e.g. platform security configuration checking and software load checking. In step  42 , the access CPU reads security hardware settings, if applicable, i.e. if such security hardware settings are present as part of a given implementation of the mobile terminal platform. Examples of such security hardware settings include One-Time-Programmable Memory, e-fuse registers etc. These settings may be used to verify security configuration and software to be loaded. 
     In step  43 , the external CPU  231  reads security configuration parameters from a non-volatile storage medium connected to the external CPU. If no memory or parameters are detected, the external CPU saves this state information in a suitable internal memory such as a RAM. 
     Subsequently, the external CPU sends the security configuration parameters  44  obtained at step  43  to the access CPU. If no configuration has been found, the external CPU informs the access CPU about this fact. 
     In step  45 , the access CPU checks the received security configuration information  44 , e.g. including integrity checking and/or the like of the received information. Subsequently, the access CPU returns an acknowledgement  46  of the receipt of the security configuration information to the external CPU. 
     Subsequently, the access CPU continues at step  47  with the boot process as described above. 
     Similarly, in step  48  the application CPU continues with the boot from the detected interface or memory. For example, at this stage, if no flash configuration has been found, the application CPU may wait for the next message from the access CPU. On the other hand, if a flash configuration has been found, the application CPU may continue to boot from the flash memory. If a bridge configuration applies, the application CPU continues the boot over the internal interface and the first code to be executed will typically shut down the application system, as the application is typically not needed in the bridge configuration where the external CPU plays the role of the application CPU. 
     In summary, described above is an embodiment of a boot procedure for a two-CPU architecture controlled by ROM-based code on both CPU systems. The first CPU (which may be an application CPU of a mobile platform system) acts as a master CPU in the boot process and the boot process includes three main steps:
         The first CPU detects possible boot configuration parameters (hardware and software) on a dedicated configuration file stored in non-volatile memory and propagates this information to the second CPU (which may be an access CPU of a mobile platform system.)   If boot configuration was found, the second CPU uses this information to boot from the correct boot interface. If no information was found by the first CPU, the second CPU searches for a connected boot interface.   Finally, the boot is continued by loading boot software from the detected boot interface and the boot software is security checked by one of the CPUs before the boot software is allowed to be executed. The security check may be based on security checking configuration information stored protected in and read at boot time from a non-volatile memory, e.g. the memory that also contains the boot configuration information.       

     Hence, the above boot process accounts for the different possible boot scenarios including non-bridge configurations, bridge configurations, service mode (i.e. for software flashing), and normal operation mode, by means of a boot interface detection procedure, e.g. as described in connection with  FIG. 4 . However, it will be appreciated that the boot sequence of  FIG. 4  can be modified, e.g. by modifying the order of detection and/or the division of tasks between the access and the application CPU. Furthermore, the boot process may also be applied to an architecture with additional CPUs. 
     In the above embodiment, the application CPU functions as a master during the initial boot process, since the application CPU generally is the CPU that has access to the memory where configuration parameters may be found. However, it will be appreciated that in alternative embodiments, the access CPU may function as a master. Furthermore, in some embodiments, the access CPU may have access to the memory including configuration parameters, if any. 
     Accordingly, although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, the boot process described herein has mainly been described in the context of a mobile platform system including an access CPU and an application CPU. It will be appreciated, however, that the boot process may also be applied to other systems, e.g. a 2-CPU mobile platform system with a different functional split between the two CPUs, or a multi-CPU processing system used for other applications than mobile communications systems. 
     The method, product means, and device described herein can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed microprocessor. In the device claims enumerating several means, several of these means can be embodied by one and the same item of hardware, e.g. a suitably programmed microprocessor, one or more digital signal processor, or the like. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage. 
     It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.