Abstract:
A n-channel memory management circuit operates as an interface unit between a microprocessor, which microprocessor is normally capable of addressing only 64K bytes of memory, to provide expandable memory configurations with a memory capacity of at least 128K bytes of read only memory (ROM) and 128K bytes of random access memory (RAM) which are directly accessed by the microprocessor in 64K bytes blocks or &#34;windows&#34; consisting of smaller size non-contiguous blocks from the entire memory configuration.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to translator circuits for accessing expanded memory locations in read only memory (ROM) and dynamic random access memory (RAM) from microprocessors originally designed to access limited electronic memory. 
     Many personal computers have been built around microprocessors such as Commodore-MOS Technology model 6502 or Zilog Corporation model Z80. These microprocessors are 8-bit devices whose internal architecture is set up to access 64K bytes of electronic memory, either RAM or ROM. These microprocessors are both reliable and inexpensive to incorporate into any computing or controller system. A limitation, however, in their use in the past has been that expanded memory, when used with these microprocessors, has had to be magnetic medium, such as magnetic tape or disk or the like. The writing and reading of data from such peripheral storage has always been slow. 
     Recent advances in manufacturing techniques has allowed the cost of electronic memory (RAM and ROM) to drop dramatically thereby making it desirable to incorporate large amounts of electronic memory into personal computers. 
     One drawback to incorporating large amounts of electronic memory into systems utilizing older design 8-bit microprocessors has been these microprocessors&#39; capabilities to direct access 64K of electronic memory. 
     It is desirable to provide a memory management unit, as a high speed interface, between large amounts of electronic memory (ROM and RAM) and the 6502 and Z80 microprocessors or similar microprocessor units, whereby these microprocessors&#39; ability to address only 64K of electronic memory is increased to plural units of 64K electronic memory. It is also desirable that such memory management unit operate in dual microprocessor systems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a memory management unit circuit as a programmable controller or translator circuit which accesses large amounts of electronic memory (RAM and ROM) in 64K units or &#34;windows&#34; from microprocessor memory addresses. 
     A second object is to provide this circuit for use in either single or plural (i.e. dual) microprocessor systems whereby the circuit is compatible with the NMOS (N-type metal oxide semiconductor) LSI (large scale integrated) circuit implementation of the host system. 
     A further object is to provide this circuit with the ability to access the 64K memory window from units of 16K memory selected from between any of a plurality of RAM and ROM chips. 
     An additional object is to select between memory size for 40 column and 80 column video display. 
     The objects of the present invention are incorporated in a circuit implemented in NMOS, LSI circuitry on a 48 pin DIP (dual in line pin) chip wherein the operating status of the circuit is switched between predetermined operating states (functions) by program instructions loaded into registers, whereby these instructions operate functionally as switches for circuit operation selection. 
     A dual microprocessor system includes an 8-bit 6502 type microprocessor (or a similar Commodore-MOS Technology 8502 microprocessor) and an 8-bit Z80 microprocessor. A 16-bit address bus is common to both microprocessors and an 8-bit data bus is likewise common to both microprocessors. A buffer circuit is used to interface the data bus to the Z80 microprocessor as the 8502 microprocessor is allowed to operate at a slower speed from the Z80. 
     A plurality of dynamic random access memory (RAM) units are available in 64K RAM chips. A plurality of read only memory (ROM) units are available in 128K ROM chips. 
     Memory addresses are received from either the 8502 or the Z80 via the address bus and translated into an expanded memory address which enables selected memory locations on the RAM chips to be available to the data bus during a particular processing cycle. Z80 and 8502 access on the data bus is controlled. 
     The expanded memory address is used to generate or control instruction which is input into a programmable logic array circuit (PLA). This PLA is implemented as a decoder circuit whose output is an exclusive control signal on one of a plurality of dedicated control lines to the 128K ROM chips. This decoder circuit output selects one of the ROM chips to be available to the data bus during a particular processing cycle. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The structural features, operation and advantages of the present invention will be better understood from a reading of the following detailed description with the accompanying drawings in which like numerals refer to like elements, and in which: 
     FIG. 1 is a circuit block diagram of a dual microprocessor system utilizing expanded electronic memory accessing in 64K memory units per processing cycle; 
     FIG. 2 is a functional operation diagram for the memory location translator component of the circuit of FIG. 1; 
     FIG. 3 is a circuit block diagram of the memory location translator component of the circuit of FIG. 1; 
     FIG. 4 is an illustration of RAMS 35 and 37 of FIG. 1; and 
     FIG. 5 is an illustration of ROM 45 of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A personal computer system 10, FIG. 1, has dual microprocessors, a Commodore-MOS Technology model 8502 microprocessor 11, and a Zilog Corporation model Z80 microprocessor 13. A keyboard 15 is connected to provide operator data and instructions to the 8502 microprocessor 11. A video/audio display 17 is connected to the Z80 microprocessor 13 or the 8502 microprocessor 11 while other peripheral devices 19, including floppy disk and paddles, are connected to both microprocessors 11, 13. An oscillator, being the system clock 21, provides 4, 2 and 1 MHz pulses, where the Z80 microprocessor 13 is operated at the 4 MHz rate and the 8502 microprocessor 11 is operated at the 2 or 1 MHz rate. 
     A 16-bit address bus 23 is connected to each of the 8502 and Z80 microprocessors 11, 13 for bi-directional transmission of addresses. An 8-bit data bus 25 is connected directly to the 8502 microprocessor 11 and to the Z80 microprocessor 13 through a data buffer 27 for bi-directional transmission of data. 
     A memory translator and control circuit 29 is connected to the data bus 25, and provides a (Z80 enable) select signal on line 31 to the Z80 microprocessor 13, its data buffer 27 and the 8502 microprocessor 11 to select between microprocessors 11 and 13 being on the system data bus 25. When the select signal is &#34;high,&#34; the Z80 microprocessor 13 is on and the 8502 microprocessor 11 is off the data bus 25. 
     The memory translator and control circuit 29 is also connected to the address bus 25. A program logic array (PLA) decoder circuit 33 is connected to the address bus 23 and to the memory translator and control circuit 29. 
     A first and second 64K dynamic random access memories (RAM) 35, 37 are each connected to the memory translator and control circuit through a multiplexer/circuit 39. The multiplexer 39 receives the 8 least significant bits 40 from the address bus 23 and 8-bit words 42 from the memory translator and control circuit 29 and transmits same to each RAM 35, 37 as 8-bit addresses. 
     An exclusive column address strobe (CAS) select signals 41, 43 is generated by the memory translator and control circuit 29 and connected to the RAMs 35, 37 with the CAS-0 signal 41 enabling the first RAM 35 and the CAS-1 signal 43 enabling the second RAM 37. Each of the RAMs 35, 37 is connected for bi-directional transmission with the data bus 25. 
     A plurality of 128K read only memories (ROM) 45 are each connected to place data on the data bus 25 when enabled by an exclusive select signal 47 from the decoder 33. Each ROM 45 is connected to the address bus 23. For the system 10, shown in FIG. 1, there are seven ROM&#39;s 45 and the decoder 33 has seven select (ROM) control lines 47. This number of ROM memories can be varied from system to system with obvious design variations in the decoder 33 to provide additional select outputs. 
     The 8502 and Z80 microprocessors 11, 13 can conceptually be seen to address large amounts of electronic memory 49, either RAM or ROM or both, through the memory translator and control circuit 29, as functionally represented by FIG. 2. As both microprocessors 11, 13 can only address 64K of memory in any processing cycle, the memory translator and control circuit 29 must translate the large amounts of memory 49 provided in the system into 64K &#34;units&#34;. Moreover, these 64K units of memory are made up into 16K non-contiguous blocks of memory from amongst the total electronic memory available 49. 
     A &#34;real&#34; memory 49 address is made up of the 8 least significant bits 40 from an address provided by either the 8502 or the Z80 microprocessors 11, 13 and 8 bits of address generated by the memory translator circuit 29 from data provided from the microprocessors 11, 13. 
     The memory translator and control circuit 29 is shown in greater detail in FIG. 3. This circuit 29 is made as an NMOS, LSI circuit component on a single 48 pin (DIP) chip. 
     Address bus 23 is connected to an address buffer 51. A chip-internal 16-bit address bus 53 is connected to lead from this address buffer 51. Data bus 25 is connected to a data buffer 55. A chip-internal 8-bit data bus 57 is connected for bi-directional data transmission with this data buffer 55. 
     Address bus 53 is connected to a ROM I/O decoder 59 and an internal address decoder 61. The eight most significant bits (A8 to A15) 60 of the address bus 53 are connected to feed into a 10-bit adder circuit 63, output of which is connected to feed onto a translated address bus 65. This translated address bus 65 is connected to a RAM control decoder 67 and to an output buffer 69, where output buffer 69 provides eight translated bits 42 to the multiplexer 39 of FIG. 1. 
     RAM control decoder 67 has as outputs the CAS-0 select signal 41 and the CAS-1 select signal 43 which are sent to the RAMs 35, 37 of FIG. 1, via a second output buffer 71. 
     Data bus 57 is connected to feed 8-bit data to a first page pointer latch register 73 (slave latch). 
     This latch register 73 is connected to transfer its contents to a second page pointer latch register 75 (master latch). The output from the second latch register 75 is ten bits of data 76 which is fed into the 10-bit adder 63. The output from the 10-bit adder 63 is fed onto the translated address bus 65. 
     The data bus 57 is also connected to feed 8-bit data to a first preconfiguration register-A 77; a second preconfiguration register-B 79; a third preconfiguration register-C 81; a fourth preconfiguration register-D 83; a configuration register 85; a mode register 87; and a RAM share register 89. 
     A 17-bit operation instruction register 95 is loaded with instruction code from the microprocessors 11, 13 via the system address and data buses 23, 25 via selected line connections. 
     Operation instruction register 95 is connected to first instruction control bus 97 which has certain of its control lines connected to the RAM control decoder 67; to the RAM I/O decoder 59; to the enable/transfer input to the second output buffer 71; to the first page pointer latch register 73; to the internal address decoder 61; and to a read/write decoder circuit 99. The identity of these control lines and the functions carried thereon will be discussed below. 
     The output from the read/write control decoder is a gated read/write signal 101 which is sent to the microprocessors 11, 13 through a third output buffer 103. 
     The operation of both the internal address decoder 61 and the read/write decoder 99 is simultaneously enabled or clocked by a single control line 105 as an output from the RAM I/O decoder 59. This control line 105 signal is present during display color, display characters and RAM selection definition operations. 
     The output from the internal address decoder 61 is a register select control signal (bus) 107 which is connected to enable or clock the operation of the configuration register 85. 
     The output from the RAM share register 89 and the mode register 87 and the configuration register 85 are connected into a second instruction control bus 109 which has certain of its control lines connected to the RAM control decoder 67; and to the ROM I/O decoder 59 as will be further identified by signal name below. 
     A third instruction control bus 111 carries seventeen instruction bits output by the ROM I/O decoder 59 to a configuration instruction register 113. The output from the configuration instruction register is connected into the PLA decoder 33 of FIG. 1. 
     A functional operation diagram for memory location accessing is shown in FIG. 4. The two RAMs 35, 37 are shown in FIG. 4 to be registerably addressable in variable-sized predetermined blocks of RAM memory as fixed and predetermined units of 1K, 4K, 8K or 16K bytes according to the instruction code illustrated, which code switches &#34;blocks&#34; to be read. 
     The first RAM 35 is normally read by the system unless the CAS-1 select 43 is activated, in which case the particular &#34;block&#34;  of RAM 35 is &#34;looked-through&#34; and the corresponding physical location in the second RAM 37 is read instead. This process works equally well for write operations. 
     The ROMs 45 are represented in FIG. 5. While the graphic representation of FIG. 5 shows only three 128K ROMs 45 from FIG. 1 in &#34;partial&#34;, seven of these ROMs 45 were discussed above in connection with the circuit of FIG. 1. 
     While each ROM 45 is 128K of memory, it can be addressed in 16K &#34;blocks&#34; as units, with the use of ROM &#34;Mid&#34; and ROM &#34;Lo&#34; instructions. A ROM &#34;Hi&#34; instruction will likewise address a 16K block 115, where in this case, the 1K, 4K, 8K and 16K bytes block instructions for accessing fixed sections of RAM also access like sized sections. 
     The memory translator and control circuit 29 operates with 17 different control signals shown labeled on FIG. 3 and identified as follows with their program instruction. 
     
         ______________________________________$FF04  LCRD    LOAD CONFIGURATION REG D$FF03  LCRC    LOAD CONFIGURATION REG C$FF02  LCRB    LOAD CONFIGURATION REG B$FF01  LCRA    LOAD CONFIGURATION REG A$FF00  LC      CONFIGURATION REGISTER$D50B  VR      VERSION REGISTER$D50A  P1H     PAGE 1 POINTER HIGH$D509  P1L     PAGE 1 POINTER LOW$D508  P0H     PAGE 0 POINTER HIGH$D507  P0L     PAGE 0 POINTER LOW$D506  RCR     RAM CONFIGURATION REGISTER$D505  MCR     MODE CONFIGURATION REGISTER$D504  PCRD    PRECONFIGURATION REGISTER D$D503  PCRC    PRECONFIGURATION REGISTER C$D502  PCRB    PRECONFIGURATION REGISTER B$D501  PCRA    PRECONFIGURATION REGISTER A$D500  CR      CONFIGURATION REGISTER______________________________________ 
    
     The four preconfiguration registers (A, B, C, D) 77, 79, 81, 83 are utilized as buffer registers for the configuration register 85. Data is loaded from one of the preconfiguration registers 77, 79, 81, 83 in the the presence of an enable instruction 91 from that register to the configuration register 85. 
     The memory translator and control circuit 29 is an 8-bit circuit where all registers, buses and latches are eight bits wide. 
     The input signals to the circuit 29 of FIG. 3 are identified as follows: 
     
         ______________________________________A0-A3, A8-A Address Lines InA4/5, A6/7  Combined Address Lines InD0-D7       Data Lines In/OutTA8-TA15    Translated Address Lines OutVcc         +5VGND         GroundPHI0        2 MHz 0 Clock InRESET       System Reset InR/W         Read/Write Line In/CAS0-/CAS1 DRAM CAS, 64K Bank OutAEC         Address Enable Control In/Z80EN      Z-80 Enable Out/GAME       GAME ROM Enable In, Control Out/EXROM      External ROM Enable In, Control OutMS0, MS1    Memory Status OutI/O SEL/    I/O Select OutC128/64     C128 or C64 Mode OutSENSE40     40/80 Status In, Control OutFSDIR       Fast Serial ENB In, DIR OutMUX         Memory Multiplex In______________________________________ 
    
     Changes can be made in the above-described invention without departing from the intent and scope thereof. The above description is therefore to be interpreted as illustrative of the invention, and is not intended to limit the invention to the embodiment(s) thereof.