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ORA indirect

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This commit is contained in:
jborza 2019-04-14 09:38:09 +02:00
parent b7b199129d
commit 686ba9b601
1 changed files with 17 additions and 12 deletions
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29
cpu.c
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@ -62,7 +62,7 @@ void ORA(State6502 * state, byte operand) {
} }
//load accumulator //load accumulator
void LDA(State6502* state, byte operand) { void LDA(State6502 * state, byte operand) {
state->a = operand; state->a = operand;
set_NV_flags(state, state->a); set_NV_flags(state, state->a);
} }
@ -81,29 +81,34 @@ word get_word(State6502 * state, word address) {
int emulate_6502_op(State6502 * state) { int emulate_6502_op(State6502 * state) {
byte* opcode = &state->memory[state->pc++]; byte* opcode = &state->memory[state->pc++];
switch (*opcode) { switch (*opcode) {
case BRK: state->running = 0; case BRK: state->running = 0;
state->flags.b = 1; state->flags.b = 1;
break; //BRK break; //BRK
case NOP: break; //NOP case NOP: break; //NOP
case ORA_IND_X: //ORA, indirect, x case ORA_IND_X: //ORA, indirect, x
unimplemented_instruction(state); //The address to be accessed by an instruction using X register indexed absolute addressing is computed by taking the 16 bit address
//from the instruction and added the contents of the X register.
//For example if X contains $92 then an STA $2000,X instruction will store the accumulator at $2092 (e.g. $2000 + $92). (STA)
{
word address_indirect = pop_word(state) + state->x;
word address = get_word(state, address_indirect);
ORA(state, state->memory[address]);
break; break;
}
case ORA_ZP: //ORA, zero page case ORA_ZP: //ORA, zero page
{ {
byte address = pop_byte(state); byte address = pop_byte(state);
ORA(state, state->memory[address]); ORA(state, state->memory[address]);
break; break;
} }
case ORA_IND_Y: case ORA_IND_Y: //ORA, indirect, y (post_indexed)
//The value in Y is added to the address at the little-endian address stored at the two-byte pair of the specified address (LSB) and the specified address plus one (MSB). {
//The value at the sum address is used to perform the computation. Indeed addressing mode actually repeats exactly the accumulator register's digits. word address_indirect = pop_word(state);
// Example word address = get_word(state, address_indirect) + state->y;
//The value $03 in Y is added to the address $C235 at addresses $002A and $002B for a sum of $C238. The value $2F at $C238 is shifted right (yielding $17) and written back to $C238. ORA(state, state->memory[address]);
//word address = pop_word(state) + state->y;
//word value = get_word(state, address);
//ORA(state, state->memory[value]);
unimplemented_instruction(state); unimplemented_instruction(state);
break; break;
}
case ORA_IMM: case ORA_IMM:
ORA(state, pop_byte(state)); ORA(state, pop_byte(state));
break; break;
@ -133,7 +138,7 @@ int emulate_6502_op(State6502 * state) {
} }
case LDA_IMM: case LDA_IMM:
{ {
LDA(state,pop_byte(state)); LDA(state, pop_byte(state));
break; break;
} }
break; break;