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Attempt fully to tie together 68020+ addressing.

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This commit is contained in:
Thomas Harte 2022-12-19 10:38:51 -05:00
parent 08b3c42a5c
commit 6832cbeb31
1 changed files with 21 additions and 29 deletions
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50
InstructionSets/M68k/Implementation/ExecutorImplementation.hpp
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@ -217,43 +217,24 @@ uint32_t Executor<model, BusHandler>::State::index_8bitdisplacement(uint32_t bas
const auto offset = int8_t(extension);
const int register_index = (extension >> 12) & 15;
// Calculate the displacement; which on the 68020+ is better known as the index.
const uint32_t raw_index = registers[register_index].l;
uint32_t index = ((extension & 0x800) ? raw_index : int16_t(raw_index)) << scale;
// Use a brief extension word if instructed to, or if that's this processor's limit.
if(!supports_full_extensions || !(extension & 0x100)) {
const uint32_t displacement = registers[register_index].l;
const uint32_t sized_displacement = (extension & 0x800) ? displacement : int16_t(displacement);
return base + offset + (sized_displacement << scale);
return base + offset + index;
}
//
// Determine a long extension.
//
// i.e. base register (BR), index register (Xn), base displacement (bd), outer displacement (od).
//
// b15: D/A (i.e. is register data or address)
// b14–12: register
// b11: W/L (i.e. word or longword use of the named register)
// b10–9: scale
// b8: 1
// b7: BS (i.e. base register suppress)
// b6: IS (i.e. index register suppress)
// b5–4: BD size (i.e. base displacement size)
// b3: 0
// b2–b0: I/IS (preindexed/postindexed/no modification of index register plus index register size)
//
enum class Displacement {
Reserved,
Null,
Word,
Long
};
enum class IndirectAction {
IndirectPreindexed,
IndirectPostindexed,
MemoryIndirect,
Reserved
};
// Apply suppressions.
const bool suppress_base = extension & 0x80; // i.e. don't use whatever the first instruction word indicated.
const bool suppress_index = extension & 0x40; // i.e. don't use whatever register_index points to.
if(suppress_base) base = 0;
if(suppress_index) index = 0;
// Fetch base displacement.
uint32_t base_displacement = 0;
@ -263,6 +244,11 @@ uint32_t Executor<model, BusHandler>::State::index_8bitdisplacement(uint32_t bas
case 3: base_displacement = read_pc<uint32_t>(); break;
}
// Don't do a further indirection if there's no outer displacement.
if(!(extension & 7)) {
return index + base + base_displacement;
}
// Fetch outer displacement.
uint32_t outer_displacement = 0;
switch(extension & 3) {
@ -271,7 +257,13 @@ uint32_t Executor<model, BusHandler>::State::index_8bitdisplacement(uint32_t bas
case 3: outer_displacement = read_pc<uint32_t>(); break;
}
return 0;
// Apply outer displacement; either the index is before the indirection
// or after it.
if(extension & 4) {
return read<uint32_t>(base + base_displacement) + index + outer_displacement;
} else {
return read<uint32_t>(base + base_displacement + index) + outer_displacement;
}
}
template <Model model, typename BusHandler>