Abstract:
A microprocessor system is provided with added memories (1-M) to expand its address spaces beyond its address word length capacity by using indirect addressing instructions of a type having a detectable operations code and dedicating designated address spaces of memory to each of the added memories, one space to a memory. By decoding each operations code of instructions read from main memory into a decoder (16) to identify indirect addressing instructions of the specified type, and then decoding the address that follows in a decoder (20) to determine which added memory is associated therewith, the associated added memory is selectively enabled through a unit (22) while the main memory is disabled to permit the instruction to be executed on the location to which the effective address of the indirect address instruction points, either before the indirect address is read from main memory or afterwards, depending on how the system is arranged by a switch (S 1 ).

Description:
ORIGIN OF INVENTION 
     The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a memory system for a microprocessor, and more particularly to expanding the address space of a microprocessor beyond the total set of 2 n  addresses, where n is the number of bits placed on an address bus to access memory. 
     A microprocessor operates with instructions from a program stored in memory, often a read-only memory (ROM), to process data stored in read/write memory, commonly referred to as a random access memory (RAM). Typically, a microprocessor operates on an 8-bit basis, although for many operations, two or more 8-bit words, commonly termed bytes, may be combined into a longer word. 
     A 16-bit address word allows access to 2 16  memory locations, which is equal to precisely 65,536 memory locations. If the microprocessor operates on an 8-bit basis, each location stores 8-bits of data or instructions. It thus follows that to read an instruction that requires an operand, three bytes must be read from memory during three successive clock pulses, a first byte consisting of the operations code (OP CODE), and two bytes consisting of a low-order address byte (LAB) and a high-order address byte (HAB). 
     The available address space is allocated between a program addressed by a P-counter, and data addressed by the instructions. As noted above, the program may be stored in a ROM (usually of moderate size--less than 8k), while the balance of the approximate total of 64k address space in a RAM is used for data. Both types of memories are addressed by the microprocessor over a 16-bit bus. 
     In some applications there is significant address space not used, which is to say that the memory is implemented with less than 2 n  storage locations. But in applications involving data acquisition, processing and storage with microprocessors, it is often desirable to have memory capacity greater than 2 n . Necessary instructions occupy much of the memory, leaving insufficient space in a memory of 2 n  locations for handling large amounts of data. To add more storage capacity, the address word must be increased such as from a 16-bit (2-byte) word to a 24-bit (3-byte) word, but that slows data handling for now four clock times are required to fetch an instruction from memory, and some of the added memory capacity is given up to storing the longer addresses. In many real-time applications, the use of longer addresses would so slow computation that the ability to keep up with data flow would be lost, computation could no longer be carried out in real-time, and incoming data could be lost. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of this invention is to provide additional memory capacity in a microprocessor system without expanding the system address space, i.e., without lengthening the address word for addressing locations of memory. 
     Still another object of this invention is to provide the capability of addressing the additional memory with minimal hardware and programming constraints. 
     Briefly, the present invention is implemented in a microprocessor system having indirect addressing instructions of a detectable type. An address in the instruction points to a location from which an effective address is normally read from the same memory from which the instruction was read. A special instruction decoder determines when an indirect addressing instruction of this detectable type is to be executed. That enables a decoder to determine if the address is associated with an added memory, and also enables an added memory select unit. The effective address may be read from the main memory, or the added memory, depending on how the system is configured, but the effective address points to a memory location in the selected added memory. All memories are connected in parallel to the address bus, but only the main memory is normally enabled. For an indirect addressing instruction of this detectable type, the main memory is disabled and the added memory selected is enabled at the appropriate time to read the effective address from the main memory or the added memory, and to address the added memory selected for the read or write operation indicated by the instruction. 
     In an exemplary microprocessor system having a 2-byte address and a 1-byte data bus connected to a main memory, the main memory space is divided into a number of pages addressed by the high-order byte of an address, each page consisting of a number of memory locations addressed by the low-order byte of an address. The maximum number of pages in the address space is 2 N , where N is the number of bits in a byte, and the maximum number of memory locations in a page is similarly 2 N , a total of 2 2N  address spaces. So in addressing main memory, the high-order byte is decoded to selectively enable a page to be accessed. The low-order byte applied to a decoder effective on all pages will then access only one memory location in the enabled page. To expand the memory capacity of the microprocessor system with a maximum of 2.sup.(N-1) added memories, where N is the number of bits in a byte, and each added memory is, like the main memory, a maximum of 2 2N  memory locations, additional memories are connected to the high-order byte detector and to the low-order byte on the address bus in parallel with the main memory. All added memories are thus addressed in parallel with the main memory, but only a selected one of the added memories is enabled for access by indirect addressing instructions of a detectable set that uses a single byte address to fetch a 2-byte effective address out of the (ZERO) page of the main memory, or of the selected added memory. In a system which does not use 2-byte addresses, and instead uses a 1 -byte address, the maximum number of added memories is 2 N  instead of 2.sup.(N-1), but then each memory has only 2 N  address spaces instead of 2 2N . 
     Each indirect addressing instruction of the detectable set consists of an OP CODE followed by a single byte ZERO page address. (The ZERO page address is generated by the OP CODE setting HAB to ZERO.) The effective address for the instruction is then read out from the ZERO page address. But if the indirect addressing instruction is of the detectable set and the effective address for the instruction is read from a predetermined section of the ZERO page, the single byte address of the indirect addressing instruction is decoded to determine that an added memory is to be accessed for the operation rather than the main memory. 
     In one embodiment, the direct address is read from the ZERO page of the added memory selected using the one-byte address of the indirect addressing instruction for reading out the low-order address byte (LAB) of the indirect address, and incrementing the one-byte address in the normal manner for all types of indirect addressing instructions to read out the high-order address byte (HAB). In another embodiment, selection of the added memory is delayed to allow the effective address to be read out of the ZERO page of the main memory, and then switching to the selected added memory for the location of the operation. This second embodiment has as an advantage over the first the ability to easily operate on the effective address. 
     The process of selecting one of the added memories deselects (disables) the main memory so that the effective low- and high-order bytes of the effective address point to a location in one of the added memories, rather than the main memory. In other words, decoding means looks at the address of the location where the effective address is stored, i.e., looks at the single byte of the indirect addressing instruction of the detectable set and determines which added memory is to be accessed by the effective address. The single-byte address following the OP CODE of one of the detectable set of indirect addressing instructions is thus the address of a location in the ZERO page and also the address of a unique one of the added memories. The operation of the instruction is then carried out using the indirect address in an added memory. The only penalty for this expanded memory is the decoding means which is comprised of means for detecting the OP CODE of an indirect addressing instruction of the detectable set, means for decoding the next byte read from memory to determine that it is an address of one of M dedicated memory locations, where M is the actual number of added memories, thus determining that the indirect addressing instruction is to operate on a location in a corresponding one of M added memories having 2 2N  address spaces, and means for selectively enabling the corresponding one of the M added memories for access with a 2-byte address, while at the same time disabling the main memory. As noted above, this switch to an added memory may be effected before or after fetching the indirect address from a ZERO page location, depending upon whether or not the user would prefer to have access to the indirect address in the main memory. 
     The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention will best be understood from the following decription when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram that illustrates both the method and implementation of the present invention. 
     FIG. 2 is a timing diagram useful in understanding the operation of the present invention. 
     FIG. 3 illustrates diagramatically the memory system involved in the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1 of the drawings, a microprocessor 10 is provided with a data bus and an address bus in the usual manner. For the purpose of illustrating the general concept of the invention, a specific example will be described based on a 8-bit microprocessor, such as the 6502 of the R6500 family of microprocessors having indirect addressing instructions that use one-byte addresses pointing to a location in the first 256 bytes of memory (ZERO page) to access 2-byte indirect addresses to any location in all available address spaces of a main memory 12, and which operates on a small byte (8-bit) basis. Consequently the data bus is 8-bits wide, the address bus is 16-bits wide, and the memory system has a maximum of 2 16  =65,536 address spaces which can be allocated between a ROM for a stored program and a RAM for data. 
     The main memory can be divided into 256 pages of 256 memory locations per page. A high-order address byte (HAB) is decoded by a page decoder 14 to enable one of the 256 pages of main memory to be accessed by the low-order address byte (LAB). The low-order byte is the only byte of address read from memory following an 8-bit OP CODE of an instruction requiring access to the ZERO page. The high-order address byte is implied by the OP CODE of such an instruction. For absolute addressing instructions a 2-byte address is read from memory immediately after the OP CODE, and for an indirect address instruction, a 2-byte effective address for the operation is read from successive memory locations in ZERO page to which the instruction points. (The single byte address of the indirect address instruction is incremented automatically to read the next ZERO page byte, the high-order byte of the effective address.) 
     The present invention utilizes indirect addressing instructions of a particular type referred to in the R6500 family of microprocessors as &#34;(INDIRECT), Y.&#34; They are all of the set with binary OP CODE XXXX0001, which renders the set easy to detect. However, as will become apparent, any detectable set of indirect addressing instructions could be used according to the present invention to provide for addressing added memories 1 through M consisting of a maximum of 2 n  memory locations for each added memory, where n is the number of address bits (16 in the exemplary embodiment disclosed) and M is an arbitrary integer equal to or less than 2.sup.(N-1) and N is the number of bits in a byte. 
     During normal operation, a program counter in the microprocessor addresses the main memory for the next instruction which is read out of memory one byte at a time. If the instruction requires memory access, more than one clock time is required to read the OP CODE, which is then stored in an instruction register, and an address which is assembled in an address register. In the usual case, two clock times are required to read out two bytes (LAB, HAB), but for an indirect address instruction, four clock times are required, one for the OP CODE, one for an address (LAB) in the ZERO page (the ZERO page is implied by the OP CODE) and two for a 2-byte (LAB, HAB) effective address. In the fifth clock time, the location to which the effective address points is operated on. The present invention provides for the effective address to point to an address in an added memory, or to be actually accessed in one of the added memories, instead of the main memory, in response to a special instruction decoder 16 enabled by a SYNC pulse to receive an OP CODE at the same time the instruction register of the microprocessor receives the OP CODE. Which added memory is to be accessed by the special indirect addressing instruction or to be accessed by the effective address of a special indirect addressing instruction, depends upon the low-order byte (LAB) address in the ZERO page being addressed by the instruction, i.e., LAB read immediately after the OP CODE, a selected code which the user programs when access to an added memory is desired. 
     In the following description of the invention, it will first be assumed that the effective address is to be fetched from an added memory. Then the alternative of fetching the effective address from main memory will be described with reference to a switch S 1  placed in its alternate position. 
     The output of the decoder 16 is a single bit which enables an 8-bit register 18 to store the next byte of the instruction read from main memory, namely the LAB of the ZERO page location used for indirect addressing. The high-order address byte (HAB) required to address the ZERO page is implied in the OP CODE, so that once the microprocessor has the LAB for addressing the ZERO page, the effective address may be read from the ZERO page, but in this case it will be in an added memory. 
     A decoder 20 decodes the LAB to determine that it is one of the ZERO page addresses dedicated to the added memories (there is one such LAB for each memory added). Immediately the output of the LAB decoder enables an added memory select unit 22 which in turn disables the main memory 12, and enables a selected added memory, much the same as the page decoder 14 enables a selected page of memory. However, it should be noted that the enable signal to the added memory selected is in addition to the page select signal applied from the page decoder to all memories in parallel. Only the one memory enabled will respond to the page select and low-order address byte (LAB) applied to all memories over the address bus. 
     The microprocessor and memory system are synchronous. The microprocessor operates with an externally generated clock pulse φ 2  shown in FIG. 2. The φ 2  clock pulses are used in the microprocessor to time its sequence control unit, which in turn provides read/write sequencing control of memory. Consequently, it is to be understood that the added decoding means comprised of the OP CODE decoder 16, 8-bit register 18, LAB decoder 20 and added memory select unit 22 also receive the system clock for synchronous operation, as well as other sequence control signals, although not shown in the drawing. For example, the special instruction decoder 16 receives the same sync control signal that the instruction register of the microprocessor receives to store the OP CODE of each instruction. It is assumed that this decoder 16 has a register for storing the OP CODE of each instruction and that the decoder then generates a single bit output only while a special indirect addressing instruction is present in the register, thus automatically clearing the single bit output when a new instruction is read out of main memory. However, in practice the decoding may be done on the OP CODE while it is on the data bus, and a single bit output stored instead, in which case the single bit is cleared after each instruction has been completed. In either case the single bit output of the decoder signals that this is a special indirect addressing instruction, and that the LAB which follows is to be stored in the register 18 for decoding by the LAB decoder 20. 
     The OP CODE of any one of the set of special indirect addressing instructions decoded enables the register 18 from after the first clock pulse of four to the next clock pulse. That enables the register 18 to store the next byte read from memory on the second clock pulse as shown in FIG. 2. The LAB is immediately decoded by the decoder 20 to enable the added memory select unit 22 to decode the LAB stored in the register 18 and enable one of said memories. Since the LAB is one of the addresses in the ZERO page dedicated to and associated with a unique one of the added memories, decoding the particular LAB indicates which added memory will be enabled. When the third and fourth clock pulses occur, the effective address is read out in two bytes. This occurs automatically, once the main memory is disabled and the added memory selected is enabled because the LAB of the instruction on the address bus is applied to all memories, and the output of the page decoders is applied to all memories. Thus all memories are addressed in parallel in the usual manner, but for a special indirect addressing instruction, the main memory 12 is disabled by the added memory select unit 22, but not until the third clock, so that in response to the third clock, the LAB of the effective address is read from added memory. The HAB of the address is then read on the fourth clock pulse, and the indicated operation (read or write) is performed on the location to which the effective address points. 
     The first embodiment of the invention illustrated in FIG. 1 by the configuration shown with the switch S 1  in the position shown provides both the effective address and the memory location for the operation in added memory. While that may have its obvious advantage of shifting more of the program burden onto the added memory, there is a disadvantage of not having the effective address readily accessible for operation on it. Since a user may want to program operations on the effective address, it may be advisable to have the effective address stored in main memory. In that case the configuration of the present invention is modified by placing the switch S 1  in its alternate position such that a 2-clock delay 24 is introduced in the enable signal for the added memory select unit. In that case the third and fourth clocks of the sequence occur while the main memory is still enabled so that the effective address is read from main memory. The delay is indicated in FIG. 2 by a broken line. At the end of that delay, the unit 22 enables an added memory selected, and disables the main memory. Consequently, until the fifth clock pulse, access is to the main memory, then the operation during the fifth clock pulse is on a location in the added memory selected by the unit 22. 
     FIG. 3 illustrates pictorially the memory system of this second embodiment. As noted hereinbefore, the main memory has a capacity of 65,536 bytes, with 256 bytes per page. Each of the added memories has an identical capacity, and in the example shown there are eight additional memories. The 6502 microprocessor addresses are called in the hexidecimal mode (base sixteen), with digit symbols 0 through 9 and A through F. For clarity of the example, the address is shown in only one byte, namely the LAB. Thus an address of 0040 represents the address number 0064 (in decimal notation) of one page. The decimal equivalents of the hexidecimal addresses are shown in parentheses in FIG. 3. 
     All addresses on the zero page (shown expanded) of the main memory are within the range of 0000 to 00FF (0000 to 0255). If an instruction is executed with an address within this range, the operation called for will be carried out unchanged on the contents of any location on that page--in the usual manner. In the standard mode of operation of the 6502, this would also be true for all indirect commands. However, the inclusion of decoding means for recognizing the special indirect addressing instructions will cause the effective address read from the ZERO page of main memory to point to a location in an added memory associated with the LAB of the instruction. This effective address may have some unique value (Y) added to it so that in fact the operation of the instruction is on the location of the effective address plus Y added to its HAB-LAB, in addition to having the effective address available in main memory for the user to program changes in that effective address. For the first embodiment (switch S 1  in position shown) the expanded ZERO page shown in FIG. 3 is not in main memory, but in the added memory selected. A section of the ZERO page shown expanded extends from 0040 to 004F (0064 to 0079, in decimal notation) for a total of eight special address locations, each dedicated to one added memory. Hence any indirect addressing OP CODE followed by an address located in this section will cause the address stored at that location to be put on the address bus. Specifically, if the command taken from a location in main memory is &#34;Indirect, Y,&#34; with the single address 0042 (0066), then the contents of location 0042-0043 on ZERO page are added to the contents of the Y register and the result is put on the address bus. If the Y register held a 0001 at the time, then the address held in 0042-0043 would be increased by one and put on the address bus. The machine would thus go to that new address in the added memory associated with 0042 to carry out the instruction. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art. Consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.