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Merge pull request #636 from TomHarte/PWM

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Attempts more accurately to match Apple's IWM windowing logic.
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Thomas Harte 2019-08-02 15:15:11 -04:00 committed by GitHub
commit 917520fb1e
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1 changed files with 31 additions and 6 deletions
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37
Components/DiskII/IWM.cpp
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@ -235,23 +235,32 @@ void IWM::run_for(const Cycles cycles) {
// Activity otherwise depends on mode and motor state. // Activity otherwise depends on mode and motor state.
int integer_cycles = cycles.as_int(); int integer_cycles = cycles.as_int();
switch(shift_mode_) { switch(shift_mode_) {
case ShiftMode::Reading: case ShiftMode::Reading: {
// Per the IWM patent, column 7, around line 35 onwards: "The expected time
// is widened by approximately one-half an interval before and after the
// expected time since the data is not precisely spaced when read due to
// variations in drive speed and other external factors". The error_margin
// here implements the 'after' part of that contract.
const auto error_margin = Cycles(bit_length_.as_int() >> 1);
if(drive_is_rotating_[active_drive_]) { if(drive_is_rotating_[active_drive_]) {
while(integer_cycles--) { while(integer_cycles--) {
drives_[active_drive_]->run_for(Cycles(1)); drives_[active_drive_]->run_for(Cycles(1));
++cycles_since_shift_; ++cycles_since_shift_;
if(cycles_since_shift_ == bit_length_ + Cycles(2)) { if(cycles_since_shift_ == bit_length_ + error_margin) {
propose_shift(0); propose_shift(0);
} }
} }
} else { } else {
while(cycles_since_shift_ + integer_cycles >= bit_length_ + Cycles(2)) { while(cycles_since_shift_ + integer_cycles >= bit_length_ + error_margin) {
const auto run_length = bit_length_ + error_margin - cycles_since_shift_;
integer_cycles -= run_length.as_int();
cycles_since_shift_ += run_length;
propose_shift(0); propose_shift(0);
integer_cycles -= bit_length_.as_int() + 2 - cycles_since_shift_.as_int();
} }
cycles_since_shift_ += Cycles(integer_cycles); cycles_since_shift_ += Cycles(integer_cycles);
} }
break; } break;
case ShiftMode::Writing: case ShiftMode::Writing:
if(drives_[active_drive_]->is_writing()) { if(drives_[active_drive_]->is_writing()) {
@ -351,12 +360,28 @@ void IWM::propose_shift(uint8_t bit) {
// TODO: synchronous mode. // TODO: synchronous mode.
// LOG("Shifting input"); // LOG("Shifting input");
// See above for text from the IWM patent, column 7, around line 35 onwards.
// The error_margin here implements the 'before' part of that contract.
//
// Basic effective logic: if at least 1 is fozund in the bit_length_ cycles centred
// on the current expected bit delivery time as implied by cycles_since_shift_,
// shift in a 1 and start a new window wherever the first found 1 was.
//
// If no 1s are found, shift in a 0 and don't alter expectations as to window placement.
const auto error_margin = Cycles(bit_length_.as_int() >> 1);
if(bit && cycles_since_shift_ < error_margin) return;
shift_register_ = uint8_t((shift_register_ << 1) | bit); shift_register_ = uint8_t((shift_register_ << 1) | bit);
if(shift_register_ & 0x80) { if(shift_register_ & 0x80) {
data_register_ = shift_register_; data_register_ = shift_register_;
shift_register_ = 0; shift_register_ = 0;
} }
cycles_since_shift_ = Cycles(0);
if(bit)
cycles_since_shift_ = Cycles(0);
else
cycles_since_shift_ -= bit_length_;
} }
void IWM::set_drive(int slot, IWMDrive *drive) { void IWM::set_drive(int slot, IWMDrive *drive) {