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Add scheduler side of PC writeback.

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This commit is contained in:
Thomas Harte 2024-02-28 10:15:23 -05:00
parent d6bf1808f9
commit 5a48c15e46
2 changed files with 48 additions and 52 deletions
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10
InstructionSets/ARM/Registers.hpp
View File

@ -63,7 +63,7 @@ struct Registers {
void begin_fiq() { interrupt_flags_ |= ConditionCode::FIQDisable; }
/// @returns The full PC + status bits.
uint32_t status(uint32_t offset) const {
uint32_t pc_status(uint32_t offset) const {
return
uint32_t(mode_) |
((active[15] + offset) & ConditionCode::Address) |
@ -74,6 +74,14 @@ struct Registers {
interrupt_flags_;
}
void set_status(uint32_t) {
// ... in user mode the other flags (I, F, M1, M0) are protected from direct change
// but in non-user modes these will also be affected, accepting copies of bits 27, 26,
// 1 and 0 of the result respectively.
}
void set_pc(uint32_t) {}
uint32_t pc(uint32_t offset) const {
return (active[15] + offset) & ConditionCode::Address;
}

82
OSBindings/Mac/Clock SignalTests/ARMDecoderTests.mm
View File

@ -110,52 +110,19 @@ struct Scheduler {
}
template <Flags f> void perform(DataProcessing fields) {
// TODO: ensure R15 is handled correctly.
//
// From the data sheet:
//
// # Writing to R15
//
// When Rd is a register other than R15, the condition code flags in the PSR may be
// updated from the ALU flags as described above. When Rd is R15 and the S flag in
// the instruction is set, the PSR is overwritten by the corresponding ALU result
// ... in user mode the other flags (I, F, M1, M0) are protected from direct change
// but in non-user modes these will also be affected, accepting copies of bits 27, 26,
// 1 and 0 of the result respectively.
//
// ...
//
// If the S flag is clear when Rd is R15, only the 24 PC bits of R15 will be written.
// Conversely, if the instruction is of a type which does not normally produce a result
// (CMP, CMN, TST, TEQ) but Rd is R15 and the S bit is set, the result will be used in
// this case to update those PSR flags which are not protected by virtue of the
// processor mode.
//
// # Using R15 as an operand
//
// When R15 appears in the Rm position it will give the value of the PC together
// with the PSR flags to the barrel shifter.
//
// When R15 appears in either of the Rn or Rs positions it will give the value
// of the PC alone, with the PSR bits replaced by zeroes.
//
// The PC value will be the address of the instruction, plus 8 or 12 bytes due to
// instruction prefetching. If the shift amount is specified in the instruction, the
// PC will be 8 bytes ahead. If a register is used to specify the shift amount, the
// PC will be 8 bytes ahead when used as Rs and 12 bytes ahead when used as Rn
// or Rm.
constexpr DataProcessingFlags flags(f);
const bool shift_by_register = !flags.operand2_is_immediate() && fields.shift_count_is_register();
auto &destination = registers_.active[fields.destination()];
// Write a raw result into the PC proxy if the target is R15; it'll be stored properly later.
uint32_t pc_proxy;
auto &destination = fields.destination() == 15 ? pc_proxy : registers_.active[fields.destination()];
// When R15 appears in either of the Rn or Rs positions it will give the value
// "When R15 appears in either of the Rn or Rs positions it will give the value
// of the PC alone, with the PSR bits replaced by zeroes. ...
//
// If the shift amount is specified in the instruction, the PC will be 8 bytes ahead.
// If a register is used to specify the shift amount, the PC will be ... 12 bytes ahead
// when used as Rn or Rm.
// when used as Rn or Rm."
const uint32_t operand1 =
(fields.operand1() == 15) ?
registers_.pc(shift_by_register ? 12 : 8) :
@ -176,11 +143,11 @@ struct Scheduler {
} else {
uint32_t shift_amount;
if(fields.shift_count_is_register()) {
// When R15 appears in either of the Rn or Rs positions it will give the value
// "When R15 appears in either of the Rn or Rs positions it will give the value
// of the PC alone, with the PSR bits replaced by zeroes. ...
//
// If a register is used to specify the shift amount, the
// PC will be 8 bytes ahead when used as Rs.
// PC will be 8 bytes ahead when used as Rs."
shift_amount =
fields.shift_register() == 15 ?
registers_.pc(8) :
@ -189,14 +156,14 @@ struct Scheduler {
shift_amount = fields.shift_amount();
}
// When R15 appears in the Rm position it will give the value of the PC together
// "When R15 appears in the Rm position it will give the value of the PC together
// with the PSR flags to the barrel shifter. ...
//
// If the shift amount is specified in the instruction, the PC will be 8 bytes ahead.
// If a register is used to specify the shift amount, the PC will be ... 12 bytes ahead
// when used as Rn or Rm.
// when used as Rn or Rm."
if(fields.operand2() == 15) {
operand2 = registers_.status(shift_by_register ? 12 : 8);
operand2 = registers_.pc_status(shift_by_register ? 12 : 8);
} else {
operand2 = registers_.active[fields.operand2()];
}
@ -273,17 +240,38 @@ struct Scheduler {
break;
}
// Set N and Z in a unified way.
if constexpr (flags.set_condition_codes()) {
registers_.set_nz(conditions);
// "When Rd is a register other than R15, the condition code flags in the PSR may be
// updated from the ALU flags as described above. When Rd is R15 and the S flag in
// the instruction is set, the PSR is overwritten by the corresponding ALU result.
//
// ... if the instruction is of a type which does not normally produce a result
// (CMP, CMN, TST, TEQ) but Rd is R15 and the S bit is set, the result will be used in
// this case to update those PSR flags which are not protected by virtue of the
// processor mode."
if(fields.destination() == 15) {
if constexpr (is_comparison(flags.operation())) {
registers_.set_status(pc_proxy);
} else {
registers_.set_status(pc_proxy);
registers_.set_pc(pc_proxy);
}
} else {
// Set N and Z in a unified way.
registers_.set_nz(conditions);
// Set C from the barrel shifter if applicable.
if constexpr (shift_sets_carry) {
registers_.set_c(rotate_carry);
}
// TODO: If register 15 was in use as a destination, write back and clean up.
}
} else {
// "If the S flag is clear when Rd is R15, only the 24 PC bits of R15 will be written."
if(fields.destination() == 15) {
registers_.set_pc(pc_proxy);
}
}
}
template <Operation, Flags> void perform(Condition, Multiply) {}