Abstract:
A computing system is presented which has a central processing unit (CPU) and peripheral devices. Identification registers associated with each peripheral device may be accessed by the CPU to identify characteristics of individual peripheral devices. Additionally, configuration registers within each peripheral device allow a particular peripheral device to determine whether instructions from the CPU are addressing that particular peripheral device.

Description:
This application is a continuation of application Ser. No. 609,376, filed 5/11/84, now abandoned. 
    
    
     BACKGROUND 
     In typical computer systems, configuration of various system components is fixed. In a typical prior art static configuration scheme for a computing system, as shown in FIG. 1, a central processing unit (CPU) 50 is coupled to peripheral devices 51, 52, 53, and 54. Each device of peripheral devices 51-54 may be, for example, a memory device such as a random access memory (RAM) or a read only memory (ROM), or a non-memory device such as a display, printer, or communication controller, Peripheral devices 51-54 are coupled to CPU 50 through a bus 65. Each of peripheral devices 51-54 has assigned to it a fixed address. CPU 50 may independently interact with each of peripheral devices 51-54 by utilizing its fixed address. Peripheral devices 51-54 are each coupled to an associated address compare register 61-64 and an address select circuit 71-74. When CPU 50 or another device sends an instruction on bus 65, address compare registers 61-64 and address select circuits 71-74 screen the instruction so that each of the peripheral devices 51-54 responds only to instruction which include the fixed address assigned to that peripheral device. 
     The use of static configuration schemes, as that described above, inherently limits flexibility. Adding or removing devices becomes difficult, as some physical change in the system is required to allow a new device to be accessed or to indicate that a removed device is no longer present. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention a computing system is presented which has a central processing unit (CPU) and peripheral devices which are dynamically configured by the CPU. Each peripheral device has a built-in identification code that identifies the characteristics of the peripheral device. The CPU can read the identification code to discover the peripheral device type (e.g., whether the peripheral device consists of a ROM, RAM, controller, or etc.) and other pertinent information about the peripheral device (e.g., memory size of a memory device). 
     Each peripheral device also contains a configuration register. The number of bits in each of the configuration registers is less than or equal to the number of bits used by the computer system for a full memory address. Each peripheral device also has a one-bit register referred to as a configuration flag. The state of the configuration flat in a particular peripheral device indicates whether that peripheral device has been configured by the CPU and thus whether the contents of the configuration register are valid. 
     After the CPU utilizes the identification code to determine the characteristics of a particular peripheral device, it uses this information to allocate address space to that peripheral device. The CPU then configures the peripheral device by writing one or more bits into the configuration register and setting the configuration flag of the peripheral device. Similarly, the CPU &#34;unconfigures&#34; or removes a peripheral device from configuration by clearing that peripheral device&#39;s configuration flag. In this way the CPU is able to dynamically adjust the configuration of the computer system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a prior art configuration scheme for a computing device. 
     FIG. 2 is a schematic of a central processing unit (CPU) coupled to a series of peripheral devices in accordance to the preferred embodiment of the present invention. 
     FIG. 3 is a schematic of a peripheral device in accordance with the preferred embodiment of the present invention. 
     FIG. 4 is a block diagram of registers within a peripheral device in accordance with the preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 2 a central processing unit (CPU) 201 is coupled to a series of peripheral devices 101, 111, 121, 131, 141, and 151. Each peripheral device 101, 111, 121, 131, 141 and 151 has an associated bus labeled 109, 119, 129, 139, 149, and 159 respectively. Busses 109, 119, 129, 139, 149, and 159 are coupled in parallel to a bus 202 which is in turn coupled to CPU 201. 
     CPU 201 sends commands and data to perhiperal devices 101, 111, 121, 131, 141, and 151 on its bus 202 which is coupled to busses 109, 119, 129, 139, 149, and 159. CPU 201 receives data from peripheral devices 101, 111, 121, 131, 141, and 151 through their respective busses 109, 119, 129, 139, 149, and 159 coupled to CPU 201&#39;s bus 202. 
     Each bus 109, 119, 129, 139, 149, and 159 is identical to each of the other busses 109, 119, 129, 139, 149, and 159. The structure of bus 109 is seen in FIG. 3. Each bus has several lines consisting of a ground line 106, a power line 105, a command line 104, a strobe line 103, and a collection of data lines 102. In the present embodiment there are four data lines in data lines 102. Ground line 106 is held at a constant voltage of 0.0 volts. Power line 105 is held at a constant voltage different from 0.0 volts, for example +4.5 volts. Command line 104 is normally driven by CPU 201 but may occasionally be driven by one or more of peripheral devices 101, 111, 121, 131, 141, or 151. Strobe line 103 is driven exclusively by CPU 201, and data lines 102 are driven by either CPU 201 or by one of peripheral devices 101, 111, 121, 131, 141, or 151. A line is at logic 0 when it is held at the same voltage as ground line 106 and is at logic 1 when it is held at the same voltage as power line 105. Strobe line 103 is asserted when it is held at logic 0 and is unasserted when it is held at logic 1. Command line 104 is asserted when it is held at logic 0 and is unasserted when it is held at logic 1. Information on data lines is data when command line is unasserted and is a command when command line is asserted. Information on data lines 102 is valid immediately prior to and during the transition of strobe line 103 from a state of assertion to a state of unassertion. 
     In FIG. 2, peripheral devices 101, 111, 121, and 131 are replaceable modules while peripheral devices 141 and 151 are permanent. Each of replaceable peripheral devices 101, 111, 121, and 131 is coupled to a daisy-chain 203 or a daisy-chain 204. In the present embodiment peripheral devices 101, 111, and 121 are coupled to daisy-chain 203 commencing at CPU 201 while peripheral device 131 is coupled to another daisy-chain 204 also commencing at CPU 201. Daisy-chain 203 commences at CPU 201, enters peripheral device 101 at daisy-chain input 107 and exits at daisy-chain output 108, enters peripheral device 111 at daisy-chain input 117 and exits at daisy-chain output 118, and enters peripheral device 121 at daisy-chain input 127 and exits at daisy-chain output 128 at which daisy-chain 203 terminates. Daisy-chain 204 commences at the CPU 201 and enters peripheral device 131 at daisy-chain input 137 and exits at daisy-chain output 138 at which daisy-chain 203 terminates. Daisy-chains 203 and 204 are driven by CPU 201, and are considered to be asserted when held at logic 1 and unasserted when held at logic 0. 
     In FIG. 4, registers within peripheral device 101 are shown. Identification code register (ID) 251, 20 bits long in the preferred embodiment, contains information identifying characteristics of peripheral device 101. CPU 201 can read ID 251 to determine the function of peripheral device 101 (e.g., whether the peripheral device 101 contains a ROM, RAM, controller, memory-mapped I/O device) and to discover other pertinent information about peripheral device 101 (e.g., the memory size). Only replaceable peripheral devices 101, 111, 121, and 131 need have an ID. Permanent peripheral devices 141 and 151 do not need to have an ID. 
     Peripheral device 101 also has a configuration register 252. Configuration register 252 contains fewer bits than the number of bits required by CPU 201 to access its entire memory space (the number of bits required by CPU 201 to access its entire memory space is hereinafter referred to as CPU address length, e.g., in the present embodiment CPU address length is 20 bits). Configuration register 252, for example, may be 12 bits long, while peripheral devices 111, 121, 131, 141, and 151 may have configuration registers containing more or fewer bits depending upon the memory size of each particular peripheral devices. The contents of configuration register 252 indicates to peripheral 101 the memory address(es) peripheral device 101 has been assigned by CPU 201. 
     A configuration flag 253 is a one-bit register which is set when CPU 201 has assigned memory address(es) to peripheral device 101 (i.e. peripheral device 101 is configured). Configuration flag 253 is reset when CPU 201 has not yet assigned memory address(es) or when CPU 201 has explicitly unassigned (i.e. unconfigured) memory addresses priorly assigned to peripheral device 101. 
     Peripheral device 101 also contains a command register 256 which contains a number of bits equivalent to the number of data lines in data lines 102, e.g., four bits in the present embodiment. Command register 256 is used to store a current command sent by CPU 201 (see Table I). Each peripheral device 111, 121, 131, 141, and 151 contains a command register. 
     Peripheral device 101 also contains a data pointer register (DP) 254 and a program counter register (PC) 255. DP 254 and PC 255 each contain a number of bits equal to the CPU address length. Each peripheral device 111, 121, 131, 141, and 151 contains a DP and may contain a PC depending upon its function. 
     If peripheral device 101 is unconfigured, CPU 201 may configure it as follows. CPU 201 assigns to peripheral device a range of one or more addresses which can be directly addressed by CPU 201. In order to do this CPU 201 first asserts daisy-chain input 107. Then CPU 201 examines ID 251 to determine the characteristics of peripheral device 101. CPU 201 issues a &#34;CONFIGURE&#34; command (i.e. sends a binary code corresponding to a CONFIGURE command, see Table I) and then transfers a configuration address, in this case 20 bits of data, to peripheral device 101. Peripheral device 101 copies the 12 high-order bits into configuration register 252 and sets flag 253, indicating that it is configured. 
     CPU 201 may also unassign (unconfigure) the memory address(es) of peripheral device 101. In order to do so CPU 201 issues a &#34;Load DP&#34; command (see Table I) and then transfers over data bus 201 a 20 bit address corresponding to the configuration address assigned to peripheral device 101. Each peripheral device 101, 111, 121, 131, 141, 151 then places this address into its DP. CPU 201 then sends an &#34;UNCONFIGURE&#34; command (see Table I). Each peripheral device compares the contents of its configuration register with a corresponding number of high order bits within its DP register. Since in this case the address contained in the DP registers corresponds to the configuration address assigned to peripheral device 101, peripheral device 101 will reset configuration flag 253. 
     In the same way as it configures and unconfigures peripheral device 101, CPU may configure and unconfigure the other peripheral devices. When configured each peripheral device has a unique configuration address, thereby allowing CPU 201 to address each of peripheral devices 101, 111, 121, 131, 141, and 151 individually. 
     When a peripheral device&#39;s configuration flag is reset, that peripheral device holds its daisy-chain output line unasserted. Thus when peripheral device 101 is unconfigured its daisy-chain output line 108 is held unasserted regardless of the value of its daisy-chain input line 107. When a peripheral device&#39;s configuration flag is set that peripheral device holds its daisy-chain output line at the same value as its daisy-chain input line. Thus when peripheral device 101 is configured (configuration flag 253 is set) its daisy-chain output line 108 is held at the same logic level as its daisy-chain input line 107. 
     Once configured, peripheral device 101 utilizes the contents of configuration register 252 to evaluate whether commands issued by CPU 201 are addressing it. For instance, when a PC READ is issued, peripheral device 101 will compare the contents of PC 255 with the contents of configuration register 252 to determine whether CPU 201 is addressing memory space within peripheral device 101. In the same way, upon the receipt of a PC READ each peripheral device 111, 121, 131, 141, and 151 will also compare the contents of its PC with the contents of its configuration register to determine whether CPU 201 is addressing it. Only the peripheral device addressed by CPU 201 will respond to commands issued by CPU 201. 
     The PC of each peripheral device 101, 111, 121, 131, 141, and 151 at any particular time all contain the same value. When CPU 201 issues a LOAD PC instruction, each peripheral device 101, 111, 121, 131, 141, and 151 simultaneously loads a new value into its PC. Similarly, the DP of each peripheral device 101, 111, 121, 131, 141, and 151 at any particular time all contain the same value. 
     Except when there is a nibble transfer across data lines 102, CPU 201 holds strobe line 102 unasserted. When making a nibble transfer across data lines 102, CPU 201 strobes (i.e. asserts then unasserts strobe line 103) as described in Table I. 
     Except when transfering command codes (i.e. binary codes, see Table I) to peripheral devices 101, 111, 121, 131, 141, and 151, CPU 201 holds command line 104 unasserted. To send a command code CPU 201 places on data lines 102 logic levels (i.e. logic 0s and logic 1s) corresponding to the command code of each command (see Table I). CPU 201 then asserts command line 104 and strobes (asserts and then unasserts) strobe line 103. Each peripheral device 101, 111, 121, 131, 141, and 151 copies the command code into its command register (e.g. command register 256 of peripheral device 101) as strobe line 103 is unasserted by CPU 201. CPU 201 then unasserts command line 104. The contents of the command register of each peripheral device 101, 111, 121, 131, 141 and 151 is interpreted by that peripheral device and will indicate how each peripheral device will respond to subsequent data on data lines 102, or whether it will apply data to data lines 102. 
     The following table gives a list of commands, their binary codes and a summary of the action take by peripheral devices 101, 111, 121, 131, 141, and 151. 
     
                                           TABLE I__________________________________________________________________________Binary Code  Command   Summary of action__________________________________________________________________________0000   NOP       All peripheral devices ignore strobe            (strobes are received by each peripheral            device on strobe line 103) until next            control code is sent.0001   ID        An unconfigured peripheral device which            sees its daisy-chain input at logic 1            sends its ID nibbles on following            strobes starting with the low-order            nibble.0010   PC READ   BUS→(PC). The peripheral device            addressed by the high-order bits of the            program counter sends data pointed to by            its PC register on each following strobe            and all peripheral devices increment            their PC registers after each strobe. A            dummy strobe will immediately follow the            issuance of PC READ.0011   DP READ   BUS→(DP). The peripheral device            addressed by the high-order bits of the            data pointer sends data pointed to by            its DP register on each following strobe            and all peripheral devices increment            their DP registers after each strobe. A            dummy strobe will immediately follow the            issuance of a DP READ.0100   PC WRITE  (PC)→BUS. The peripheral device            addressed by the high-order bits of the            program counter loads data on following            strobes into a location pointed to by            its PC register and all peripheral            devices increment their PC registers            after each strobe.0101   DP WRITE  (DP)→BUS The peripheral device            addressed by the high-order bits of the            data pointer loads data on following            strobes into the a location pointed to            by its DP register and all peripheral            devices increment their DP register            after each strobe.0110   LOAD PC   BUS→PC. All peripheral devices load            the data on following strobes into their            PC registers starting with the low-order            nibble. After all 5 nibbles are trans-            ferred the command code is automically            changed to a PC READ (0010).0111   LOAD DP   BUS→DP. All peripheral devices load            the data on following strobes into their            DP registers starting with the low-order            nibble. After all 5 nibbles are trans-            ferred the command code is automically            changed to a DP READ (0011).1000   CONFIGURE The unconfigured peripheral device that            sees its DAISY-CHAIN input high loads            the following five data nibbles into its            configuration register starting with the            low-order nibble.1001   UNCONFIGURE            The peripheral device currently addres-            sed by its data pointer will unconfigure            itself. The device will then respond to            CONFIGURE and ID bus commands only. The            local DP must be loaded immediately            preceding an UNCONFIGURE command.1010   POLL      All peripheral devices that require            service pull one data line high during            the next strobe low.1011   Reserved1100   BUSCC     The peripheral device currently addres-            sed by its local DP will perform a            specialized operation as defined by the            individual peripheral device1101   Reserved1110   SHUTDOWN  Each peripheral device responds to this            command based on its own specialized            requirements.1111   RESET     All peripheral devices perform local            reset, including resetting their            configuration flags if they are replace-            able peripheral devices__________________________________________________________________________