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Add new file for opcode disassembly
This commit is contained in:
Peter Evans
parent
1942a75d9e
commit
32d08fbbc5
186
src/mos6502.dis.c
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186
src/mos6502.dis.c
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/*
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* mos6502.dis.c
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*
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* Disassembly of the mos6502 machine code into an assembly notation.
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*/
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#include "mos6502.h"
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#include "mos6502.enums.h"
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static char *instruction_strings[] = {
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"ADC",
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"AND",
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"ASL",
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"BCC",
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"BCS",
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"BEQ",
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"BIT",
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"BMI",
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"BNE",
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"BPL",
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"BRK",
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"BVC",
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"BVS",
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"CLC",
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"CLD",
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"CLI",
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"CLV",
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"CMP",
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"CPX",
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"CPY",
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"DEC",
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"DEX",
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"DEY",
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"EOR",
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"INC",
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"INX",
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"INY",
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"JMP",
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"JSR",
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"LDA",
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"LDX",
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"LDY",
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"LSR",
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"NOP",
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"ORA",
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"PHA",
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"PHP",
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"PLA",
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"PLP",
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"ROL",
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"ROR",
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"RTI",
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"RTS",
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"SBC",
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"SEC",
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"SED",
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"SEI",
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"STA",
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"STX",
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"STY",
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"TAX",
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"TAY",
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"TSX",
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"TXA",
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"TXS",
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"TYA",
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};
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/*
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* Given a stream, address mode and 16-bit value, print the value out in
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* the form that is expected given the address mode. The value is not
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* necessarily going to truly be 16-bit; most address modes use one
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* 8-bit operand. But we can contain all possible values with the 16-bit
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* type.
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*/
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void
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mos6502_dis_operand(FILE *stream, int addr_mode, vm_16bit value)
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{
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switch (addr_mode) {
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case ACC:
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break;
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case ABS:
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fprintf(stream, "$%04X", value);
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break;
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case ABX:
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fprintf(stream, "$%04X,X", value);
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break;
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case ABY:
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fprintf(stream, "$%04X,Y", value);
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break;
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case IMM:
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fprintf(stream, "#$%02X", value);
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break;
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case IMP:
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break;
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case IND:
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fprintf(stream, "($%04X)", value);
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break;
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case IDX:
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fprintf(stream, "($%02X,X)", value);
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break;
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case IDY:
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fprintf(stream, "($%02X),Y", value);
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break;
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case REL:
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// FIXME: we need some kind of table of jumps and branches
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// we make, so that we can come up with some labels to use.
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fprintf(stream, "(REL)");
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break;
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case ZPG:
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fprintf(stream, "$%02X", value);
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break;
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case ZPX:
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fprintf(stream, "$%02X,X", value);
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break;
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case ZPY:
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fprintf(stream, "$%02X,Y", value);
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break;
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}
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}
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/*
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* This function will write to the stream the instruction that the given
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* opcode maps to.
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*/
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void
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mos6502_dis_instruction(FILE *stream, vm_8bit opcode)
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{
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int inst_code;
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inst_code = mos6502_instruction(opcode);
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// Arguably this could or should be done as fputs(), which is
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// presumably a simpler output method. But, since we use fprintf()
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// in other places, I think we should continue to do so. Further, we
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// use a simple format string (%s) to avoid the linter's complaints
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// about potential security issues.
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fprintf(stream, "%s", instruction_strings[inst_code]);
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}
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/*
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* This function returns the number of bytes that the given opcode is
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* expecting to work with. For instance, if the opcode is in absolute
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* address mode, then we will need to read the next two bytes in the
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* stream to compose a full 16-bit address to work with. If our opcode
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* is in immediate mode, then we only need to read one byte. Many
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* opcodes will read no bytes at all from the stream (in which we return
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* zero).
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*/
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int
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mos6502_dis_expected_bytes(vm_8bit opcode)
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{
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int addr_mode = mos6502_addr_mode(opcode);
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switch (addr_mode) {
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// These are 16-bit operands, because they work with absolute
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// addresses in memory.
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case ABS:
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case ABY:
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case ABX:
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case IND:
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return 2;
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// These are the 8-bit operand address modes.
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case IMM:
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case IDX:
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case IDY:
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case REL:
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case ZPG:
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case ZPX:
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case ZPY:
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return 1;
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// These two address modes have implied arguments; ACC is
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// the accumulator, and IMP basically means it operates on
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// some specific (presumably obvious) thing and no operand
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// is necessary.
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case ACC:
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case IMP:
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return 0;
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}
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// I don't know how we got here, outside of foul magicks and cruel
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// trickery. Let's fearfully return zero!
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return 0;
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}
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