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correcting adder/subtractor

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Thiago Auler 2017-11-22 13:18:54 -02:00 committed by GitHub
parent dd4a315e93
commit 258a9f6692
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1 changed files with 9 additions and 17 deletions
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26
src/opcodes.c
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@ -102,13 +102,13 @@ void adjustNZ(db r)
db adder(db a, db b) db adder(db a, db b)
{ {
db r = a + b; db r = a + b + C_IS_SET;
db c = (a + b) >> 8; db c = (a + b + C_IS_SET) >> 8;
a = a & 0b01111111; a = a & 0x7F;
b = b & 0b01111111; b = b & 0x7F;
db cc = (a + b) >> 7; db z = (a + b + C_IS_SET) >> 7;
db v = c ^ cc; db v = c ^ z;
if (c == 1) { C_SET; } else { C_UNSET;} if (c == 1) { C_SET; } else { C_UNSET;}
if (v == 1) { V_SET; } else { V_UNSET; } if (v == 1) { V_SET; } else { V_UNSET; }
@ -118,15 +118,6 @@ db adder(db a, db b)
return r; return r;
} }
db subtractor(db a, db b)
{
// negate b operand using 2's complement
b = b ^ 0xFF;
b = b + 1;
return adder(a, b);
}
void push_byte(db data) void push_byte(db data)
{ {
write_mem(sp, data); write_mem(sp, data);
@ -161,7 +152,7 @@ void adc()
{ {
// add memory to accumulator with carry // add memory to accumulator with carry
fetch_operand(); fetch_operand();
ac = adder(ac, operand + C_IS_SET); ac = adder(ac, operand);
} }
void and() void and()
@ -557,7 +548,8 @@ void sbc()
{ {
// subtract memory from accumulator with borrow // subtract memory from accumulator with borrow
fetch_operand(); fetch_operand();
ac = subtractor(ac, operand - C_IS_SET); operand = operand ^ 0xFF;
ac = adder(ac, operand);
} }
void sec() void sec()