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Better info on each opcode sequence

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Plus fix a display bug that showed the operand in the wrong order.
This commit is contained in:
Peter Evans 2018-01-08 21:21:09 -06:00
parent 7c899122ad
commit e4049a6a11
1 changed files with 35 additions and 19 deletions
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54
src/mos6502.dis.c
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@ -129,7 +129,9 @@ mos6502_dis_operand(mos6502 *cpu,
mos6502_dis_label(stream, rel_address);
break;
case ZPG:
fprintf(stream, "$%02X", value);
// We add a couple of spaces here to help our output
// comments line up.
fprintf(stream, "$%02X ", value);
break;
case ZPX:
fprintf(stream, "$%02X,X", value);
@ -236,21 +238,16 @@ mos6502_dis_opcode(mos6502 *cpu, FILE *stream, int address)
switch (expected) {
case 2:
// If we have a two-byte operand, then the first byte is the
// MSB and our operand will need to shift it 8 positions to
// the left before it can be OR'd.
operand |= vm_segment_get(cpu->memory, address) << 8;
address++;
// Note we fall through here to the next case...
// Remember that the 6502 is little-endian, so the operand
// needs to be retrieved with the LSB first and the MSB
// second.
operand |= vm_segment_get(cpu->memory, address++);
operand |= vm_segment_get(cpu->memory, address++) << 8;
break;
case 1:
// If it's a one-byte operand, then this byte should occupy
// the LSB space which simply requires we OR the byte
// directly in. If this is part of a two-byte operand, then
// the same logic still applies.
operand |= vm_segment_get(cpu->memory, address);
address++;
operand |= vm_segment_get(cpu->memory, address++);
break;
// And, in any other case (e.g. 0), we are done; we don't
// read anything, and we leave the operand as it is.
@ -289,14 +286,33 @@ mos6502_dis_opcode(mos6502 *cpu, FILE *stream, int address)
// Print out the instruction code that our opcode represents.
mos6502_dis_instruction(stream, inst_code);
if (expected) {
// Let's "tab" over; each instruction code is 3 characters, so let's
// move over 5 spaces (4 spaces indent + 1, just to keep everything
// aligned by 4-character boundaries).
fprintf(stream, " ");
// Let's "tab" over; each instruction code is 3 characters, so let's
// move over 5 spaces (4 spaces indent + 1, just to keep everything
// aligned by 4-character boundaries).
fprintf(stream, " ");
if (expected) {
// Print out the operand given the proper address mode.
mos6502_dis_operand(cpu, stream, address, addr_mode, operand);
} else {
// Print out a tab to get a consistent look in our
// disassembled code (e.g. to take up the space that an
// operand would otherwise occupy).
fprintf(stream, "\t");
}
// Here we just want to show a few pieces of information; one,
// what the PC was at the point of this opcode sequence; two,
// the opcode;
fprintf(stream, "\t; pc=$%02x%02x: %02x",
cpu->PC >> 8, cpu->PC & 0xff, opcode);
// And three, the operand, if any. Remembering that the operand
// should be shown in little-endian order.
if (expected == 2) {
fprintf(stream, " %02x %02x", operand & 0xff, operand >> 8);
} else if (expected == 1) {
fprintf(stream, " %02x", operand & 0xff);
}
// And let's terminate the line.