LIB6502(3)               BSD Library Functions Manual               LIB6502(3)

     lib6502 - 6502 microprocessor emulator

     #include <stdint.h>
     #include <lib6502.h>

     M6502 *
     M6502_new(M6502_Registers *registers, M6502_Memory memory,
             M6502_Callbacks *callbacks);

     M6502_reset(M6502 *mpu);

     M6502_nmi(M6502 *mpu);

     M6502_irq(M6502 *mpu);

     M6502_getVector(M6502 *mpu, vector);

     M6502_setVector(M6502 *mpu, vector, uint16_t address);

     M6502_getCallback(M6502 *mpu, type, uint16_t address);

     M6502_setCallback(M6502 *mpu, type, uint16_t address,
             M6502_Callback callback);

     M6502_run(M6502 *mpu);

     M6502_disassemble(M6502 *mpu, uint16_t addres_s, char buffer[64]);

     M6502_dump(M6502 *mpu, char buffer[64]);

     M6502_delete(M6502 *mpu);

     M6502_new() creates an instance of a 6502 microprocessor.  M6502_reset(),
     M6502_nmi() and M6502_irq() place it into the states associated with the
     hardware signals for reset, non-maskable interrupt and interrupt request,
     respectively.  The macros M6502_getVector() and M6502_setVector() read
     and write the vectors through which the processor jumps in response to
     the above signals.  The macros M6502_getCallback() and M6502_setVecttor()
     read and write client-supplied functions that intercept accesses to mem-
     ory.  M6502_run() begins emulated execution.  M6502_dump() and
     M6502_disassemble() create human-readable representations of processor or
     memory state.  M6502_delete() frees all resources associated with a pro-
     cessor instance.  Each of these functions and macros is described in more
     detail below.

     M6502_new() returns a pointer to a M6502 structure containing at least
     the following members:

     struct _M6502
         M6502_Registers  *registers;   /* processor state */
         uint8_t          *memory;      /* memory image */
         M6502_Callbacks  *callbacks;   /* r/w/x callbacks */

     These members are initialised according to the supplied registers, memory
     and callbacks arguments.  If a given argument is NULL, the corresponding
     member is initialised automatically with a suitable (non-NULL) value.

     The members of M6502 are as follows:

     registers  the processor state, containing all registers and condition

     memory     a block of at least 64 kilobytes of storage containing the
                processor's memory.  (An array type M6502_Memory, suitable for
                defining values to pass as the memory argument, is defined in
                the #include <lib6502.h>
                include file.)

     callbacks  a structure mapping processor memory accesses to client call-
                back functions.

     Access to the contents of the registers and memory members can be made
     directly.  The registers member is a M6502_Registers containing the fol-
     lowing members:

     struct _M6502_Registers
         uint8_t   a;  /* accumulator */
         uint8_t   x;  /* X index register */
         uint8_t   y;  /* Y index register */
         uint8_t   p;  /* processor status register */
         uint8_t   s;  /* stack pointer */
         uint16_t pc;  /* program counter */

     The memory member is an array of unsigned char and can be indexed
     directly.  In addition, two convenience macros M6502_getVector() and
     M6502_setVector() provide access to the reset and interrupt vectors
     within memory.  M6502_getVector() returns the address stored in the named
     vector which must be precisely one of the following:

           RST  the reset vector.

           NMI  the non-maskable interrupt vector.

           IRQ  the interrupt request vector.

     M6502_setVector() stores its address argument in the named vector and
     returns the new value.

     The callbacks member contains an opaque structure mapping processor mem-
     ory accesses to client callback functions.  Whenever the processor per-
     forms an access for which a corresponding entry exists in the the
     callbacks structure, the emulator suspends execution and invokes the
     callback to complete the operation.  Each callback function should have a
     signature equivalent to:

           int callback (M6502 *mpu, uint16_t address, uint8_t data);

     The macros M6502_getCallback() and M6502_setCallback() read and write
     entries in the callbacks structure.  These macros identify a unique mem-
     ory access operation from the specified address on which it operates and
     type of access involved.  The type argument must be one of the following:

     read   the callback is invoked when the processor attempts to read from
            the given address.  The emulator passes the effective address of
            the operation to the callback in its address argument.  (The data
            argument is undefined.)  The value returned by the callback will
            be used by the emulator as the result of the read operation.

     write  the callback is invoked when the processor attempts to write to
            the given address.  The emulator passes the effective address of
            the operation to the callback in its address argument and the byte
            being written in the data argument.  The emulator will not perform
            the write operation before invoking the callback; if the write
            should complete, the callback must modify the processor's memory
            explicitly.  The valued returned from the callback is ignored.

     call   the callback is invoked when the processor attempts to transfer
            control to the given address by any instruction other than a rela-
            tive branch.  The emulator passes the destination address to the
            callback in its address argument and the instruction that initi-
            ated the control transfer in its data argument (one of JMP, JSR,
            BRK, RTS or RTI).  If the callback returns zero (the callback
            refuses to handle the operation) the emulator will allow the oper-
            ation to complete as normal.  If the callback returns a non-zero
            address (indicating that the callback has handled the operation
            internally) the emulator will transfer control to that address.

     M6502_getCallback() returns zero if there is no callback associated with
     the given type and address.  Passing zero as the callback argument of
     M6502_setCallback() removes any callback that might have been associated
     with type and address.

     M6502_run() emulates processor execution in the given mpu by repeatedly
     fetching the instruction addressed by pc and dispatching to it.  This
     function normally never returns.

     M6502_dump() writes a (NUL-terminated) symbolic representation of the
     processor's internal state into the supplied buffer.  Typical output

           PC=1010 SP=01FE A=0A X=5B Y=00 P=D1 NV-B---C

     M6502_disassemble() writes a (NUL-terminated) symbolic representation of
     the instruction in the processor's memory at the given address into the
     supplied buffer.  It returns the size (in bytes) of the instruction.  (In
     other words, the amount by which address should be incremented to arrive
     at the next instruction.)  Typical output resembles:

           1009 cpx #5B

     (The buffer arguments are oversized to allow for future expansion.)

     M6502_delete() frees the resources associated with the given mpu. Any
     members that were allocated implicitly (passed as NULL to M6502_new())
     are deallocated.  Members that were initialised from non-NULL arguments
     are not deallocated.

     You can share the memory and callbacks members of M6502 between multiple
     instances to simulate multiprocessor hardware.

     M6502_new() returns a pointer to a M6502 structure.  M6502_getVector()
     and M6502_setVector() return the contents of the given vector.
     M6502_getCallback() and M6502_setCallback() return the M6502_Callback
     function associated with the given address and access type.
     M6502_disassemble() returns the size (in bytes) of the instruction at the
     given address.  M6502_reset(), M6502_nmi(), M6502_irq(), M6502_run(),
     M6502_dump() and M6502_delete() don't return anything (unless you forgot
     to include

     The following program creates a 6502 processor, sets up callbacks for
     printing characters and halting after a BRK instruction, stores a program
     into memory that prints the alphabet, disassembles the program on stdout,
     and then executes the program.

           #include <stdint.h>
           #include <stdlib.h>
           #include <stdio.h>

           #include "lib6502.h"

           #define WRCH    0xFFEE

           int wrch(M6502 *mpu, uint16_t address, uint8_t data)
             int pc;
             pc  = mpu->memory[++mpu->registers->s + 0x100];
             pc |= mpu->memory[++mpu->registers->s + 0x100] << 8;
             return pc + 1;  /* JSR pushes next insn addr - 1 */

           int done(M6502 *mpu, uint16_t address, uint8_t data)
             char buffer[64];
             M6502_dump(mpu, buffer);
             printf("\nBRK instruction\n%s\n", buffer);

           int main(int argc, char **argv)
             M6502    *mpu = M6502_new(0, 0, 0);
             unsigned  pc  = 0x1000;

             mpu->callbacks->call[WRCH] = wrch;     /* write character */
             mpu->callbacks->call[0000] = done;     /* reached after BRK */

           # define gen1(X)        (mpu->memory[pc++] = (uint8_t)(X))
           # define gen2(X,Y)      gen1(X); gen1(Y)
           # define gen3(X,Y,Z)    gen1(X); gen2(Y,Z)

             gen2(0xA2, 'A'     );  /* LDX #'A'   */
             gen1(0x8A          );  /* TXA        */
             gen3(0x20,0xEE,0xFF);  /* JSR FFEE   */
             gen1(0xE8          );  /* INX        */
             gen2(0xE0, 'Z'+1   );  /* CPX #'Z'+1 */
             gen2(0xD0, -9      );  /* BNE 1002   */
             gen2(0xA9, '\n'    );  /* LDA #'\n'  */
             gen3(0x20,0xEE,0xFF);  /* JSR FFEE   */
             gen2(0x00,0x00     );  /* BRK        */

               uint16_t ip = 0x1000;
               while (ip < pc)
                   char insn[64];
                   ip += M6502_disassemble(mpu, ip, insn);
                   printf("%04X %s\n", ip, insn);

             M6502_setVector(mpu, RST, 0x1000);


             return 0;

     If M6502_new() cannot allocate sufficient memory it prints "out of mem-
     ory" to stderr and exits with a non-zero status.

     If M6502_run() encounters an illegal or undefined instruction, it prints
     "undefined instruction" and the processor's state to stderr, then exits
     with a non-zero status.

     M6502 is a generic name. The initial letter is mandated by C naming con-
     ventions and chosen in deference to MOS Technology, the original design-
     ers of the processor.  To the best of my knowledge the 'M' prefix was
     never stamped on a physical 6502.

     The emulator implements the CMOS version of the processor (NMOS bugs in
     effective address calculations involving page boundaries are corrected)
     but does not tolerate the execution of undefined instructions (which were
     all no-ops in the first-generation CMOS hardware).  It would be nice to
     support the several alternative instruction sets (model-specific undocu-
     mented instructions in NMOS models, and various documented extensions in
     the later CMOS models) but there are currently no plans to do so.

     The emulated 6502 will run much faster than real hardware on any modern
     computer.  The fastest 6502 hardware available at the time of writing has
     a clock speed of 14 MHz.  On a 2 GHz PowerPC, the emulated 6502 runs at
     almost 300 MHz.


     For development tools, documentation and source code:

     The software and manual pages were written by Ian Piumarta.

     The software is provided as-is, with absolutely no warranty, in the hope
     that you will enjoy and benefit from it.  You may use (entirely at your
     own risk) and redistribute it under the terms of a very liberal license
     that does not seek to restrict your rights in any way (unlike certain so-
     called 'open source' licenses that significantly limit your freedom in
     the name of 'free' software that is, ultimately, anything but free).  See
     the file COPYING for details.

     M6502_getVector() and M6502_setVector() evaluate their arguments more
     than once.

     The out-of-memory condition and attempted execution of illegal/undefined
     instructions should not be fatal errors.

     There is no way to limit the duration of execution within M6502_run() to
     a certain number of instructions or cycles.

     The emulator should support some means of implicit interrupt generation,
     either by polling or in response to (Unix) signals.

     The COMPATIBILITY section in this manual page has been diverted from its
     legitimate purpose.

     The plural of 'callback' really aught to be 'callsback'.

     Please send bug reports (and feature requests) to the author at: first-
     Name (at) lastName (dot) com.  (See AUTHORS above for suitable values of
     firstName and lastName.)

BSD                            October 31, 2005                            BSD