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6502-cpp
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Fix handling of 16 bit C,N,Z flags

This commit is contained in:
Jason Turner 2021-05-12 09:38:18 -06:00
parent a6f012878e
commit 4cbdae3ffe
1 changed files with 3 additions and 46 deletions
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49
src/6502-c++.cpp
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@ -259,7 +259,7 @@ void fixup_16_bit_N_Z_flags(std::vector<mos6502> &instructions)
// assuming A contains higher order byte and X contains lower order byte
instructions.emplace_back(ASMLine::Type::Directive, "; BEGIN remove if next is lda");
instructions.emplace_back(ASMLine::Type::Directive, "; set CPU flags assuming A holds the higher order byte already");
std::string set_flag_label = "flags_set_after_16_bit_op_" + std::to_string(instructions.size());
std::string set_flag_label = "fixup_16_bit_op_flags" + std::to_string(instructions.size());
// if high order is negative, we know it's not 0 and it is negative
instructions.emplace_back(mos6502::OpCode::bmi, Operand(Operand::Type::literal, set_flag_label));
// if it's not 0, then branch down, we know the result is not 0 and not negative
@ -268,8 +268,8 @@ void fixup_16_bit_N_Z_flags(std::vector<mos6502> &instructions)
instructions.emplace_back(mos6502::OpCode::txa);
// if low order is not negative, we know it's 0 or not 0
instructions.emplace_back(mos6502::OpCode::bpl, Operand(Operand::Type::literal, set_flag_label));
// if low order byte is negative, shift right by one bit, then we'll get the proper Z/N flags
instructions.emplace_back(mos6502::OpCode::lsr);
// if low order byte is negative, just load 1, this will properly set the Z flag and leave C correct
instructions.emplace_back(mos6502::OpCode::lda, Operand(Operand::Type::literal, "#1"));
instructions.emplace_back(ASMLine::Type::Label, set_flag_label);
instructions.emplace_back(ASMLine::Type::Directive, "; END remove if next is lda, bcc, bcs");
}
@ -810,43 +810,7 @@ bool fix_long_branches(std::vector<mos6502> &instructions, int &branch_patch_cou
}
bool fix_overwritten_flags(std::vector<mos6502> &instructions)
{
// return false;
if (instructions.size() < 3) {
return false;
}
for (size_t op = 0; op < instructions.size(); ++op) {
if (instructions[op].is_comparison) {
auto op2 = op + 1;
while (op2 < instructions.size()
&& !instructions[op2].is_comparison
&& !instructions[op2].is_branch) {
++op2;
}
if (op2 < instructions.size()
&& (op2 - op) > 1
&& instructions[op2 - 1].opcode != mos6502::OpCode::plp) {
if (instructions[op2].is_comparison) {
continue;
}
if (instructions[op2].is_branch) {
// insert a pull of processor status before the branch
instructions.insert(std::next(std::begin(instructions), static_cast<std::ptrdiff_t>(op2)), mos6502(mos6502::OpCode::plp));
// insert a push of processor status after the comparison
instructions.insert(std::next(std::begin(instructions), static_cast<std::ptrdiff_t>(op + 1)), mos6502(mos6502::OpCode::php));
return true;
}
}
}
}
return false;
}
void run(const Personality &personality, std::istream &input)
{
@ -1008,13 +972,6 @@ void run(const Personality &personality, std::istream &input)
}
}
// it seems that with the move to AVR for the base, this
// fixup no longer makes sense, but I'm not going to remove it just yet
// until we have more complex C++ examples working
// while (fix_overwritten_flags(new_instructions)) {
// do it however many times it takes
// }
while (optimize(new_instructions)) {
// do it however many times it takes
}