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Start arrangements for full extension words.

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This commit is contained in:
Thomas Harte 2022-11-30 16:21:35 -05:00
parent 3f3c9f7491
commit 95c526d957
1 changed files with 22 additions and 5 deletions
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27
InstructionSets/M68k/Implementation/ExecutorImplementation.hpp
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@ -201,14 +201,31 @@ template <typename IntT> IntT Executor<model, BusHandler>::State::read_pc() {
template <Model model, typename BusHandler>
uint32_t Executor<model, BusHandler>::State::index_8bitdisplacement() {
// TODO: if not a 68000, check bit 8 for whether this should be a full extension word;
// also include the scale field even if not.
// Get the brief extension word.
const auto extension = read_pc<uint16_t>();
// The 68000, 68080 and 68010 do not support the scale field, and are limited
// to brief extension words.
const int scale = model <= Model::M68010 ? 0 : (extension >> 9) & 3;
// Decode brief instruction word fields.
const auto offset = int8_t(extension);
const int register_index = (extension >> 12) & 15;
const uint32_t displacement = registers[register_index].l;
const uint32_t sized_displacement = (extension & 0x800) ? displacement : int16_t(displacement);
return offset + sized_displacement;
// The 68000, 68080 and 68010 support only brief extension words.
if(model <= Model::M68010 || !(extension & 0x100)) {
const uint32_t displacement = registers[register_index].l << scale;
const uint32_t sized_displacement = (extension & 0x800) ? displacement : int16_t(displacement);
return offset + sized_displacement;
}
// Determine a long extension.
//
// Cf. https://edwardhalferty.com/2020/12/16/decoding-the-extended-addressing-modes-of-the-68000/
// const bool has_base = extension & 0x80;
// const bool has_index = extension & 0x40;
return 0;
}
template <Model model, typename BusHandler>