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Resolver may be NULL; don't call if so

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This change also registers a default value for operand (0).
This commit is contained in:
Peter Evans 2018-01-05 14:57:38 -06:00
parent d5b6205713
commit c7aa9e9fc3
1 changed files with 8 additions and 2 deletions
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8
src/mos6502.c
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@ -352,7 +352,7 @@ mos6502_get_instruction_handler(vm_8bit opcode)
void
mos6502_execute(mos6502 *cpu, vm_8bit opcode)
{
vm_8bit operand;
vm_8bit operand = 0;
int cycles;
mos6502_address_resolver resolver;
mos6502_instruction_handler handler;
@ -369,7 +369,13 @@ mos6502_execute(mos6502 *cpu, vm_8bit opcode)
// side-effect, resolver will set the last_addr field in cpu to the
// effective address where the operand can be found in memory, or
// zero if that does not apply (such as in immediate mode).
//
// Note also that resolver may be NULL, as there may not be any
// operand for this instruction! If so, we let the default for
// operand stand, which is zero.
if (resolver) {
operand = resolver(cpu);
}
// Here's where the magic happens. Whatever the instruction does, it
// happens in the handler function.