mirror of
https://github.com/JorjBauer/aiie.git
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139 lines
3.0 KiB
C++
139 lines
3.0 KiB
C++
#include "noslotclock.h"
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#include "applemmu.h" // for FLOATING
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#define initSequence 0x5CA33AC55CA33AC5LL
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/* The no-slot clock works like this...
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*
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* The NSC is installed in some bank of ROM memory. For our instance,
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* it's 0xC300 or 0xC800. When a read or write occurs in these pages
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* - and ROM isn't switched out or whatever - then this driver is
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* invoked.
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*
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* (It's the job of applemmu to decide if the right address is being
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* called to invoke the read or write here.)
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*
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* To get the clock to respond, we need to first pass it the init
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* Sequence (above). The NSC gets one bit at a time from watching the
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* transactions on the bus. So first the driver reads or writes to
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* memory address (e.g.) 0xC800 or 0xC801, depending on whether it
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* wants to send a 0 or 1 bit; and then, if the NSC sees all of the
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* correct bits for the init sequence, it allows responses when
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* reading from memory (e.g.) 0xC804. These responses are, again, one
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* bit at a time of the current date and time.
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*/
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NoSlotClock::NoSlotClock(AppleMMU *mmu)
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{
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this->mmu = mmu;
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compareReg = initSequence;
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clockReg = 0x00LL;
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clockRegPtr = compareRegPtr = 0;
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regEnabled = false;
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writeEnabled = true;
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}
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NoSlotClock::~NoSlotClock()
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{
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}
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bool NoSlotClock::read(uint8_t s, uint8_t *d)
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{
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if (s & 0x04) {
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return doRead(d);
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}
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else {
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doWrite(s);
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return false;
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}
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}
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void NoSlotClock::write(uint8_t s)
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{
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if (s & 0x04) {
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doRead(0);
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} else {
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doWrite(s);
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}
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}
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bool NoSlotClock::doRead(uint8_t *d)
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{
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if (!regEnabled) {
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compareReg = initSequence;
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compareRegPtr = 0;
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writeEnabled = true;
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return false;
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}
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if (d) {
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*d = (clockReg & 0x01) ?
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((*d) | 1) :
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((*d) & ~1);
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}
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clockRegPtr++;
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clockReg >>= 1;
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if (clockRegPtr == 64) {
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regEnabled = false;
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clockRegPtr = 0;
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}
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return true;
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}
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void NoSlotClock::doWrite(uint8_t address)
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{
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if (!writeEnabled) {
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return;
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}
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if (!regEnabled) {
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if ((compareReg & 0x01) == (address & 0x01)) {
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compareRegPtr++;
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compareReg >>= 1;
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if (compareRegPtr == 64) {
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regEnabled = true;
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compareRegPtr = 0;
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compareReg = initSequence;
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populateClockRegister();
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}
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} else {
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writeEnabled = false;
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}
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} else {
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// The NSC driver is writing a new clock time to the clock...
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clockRegPtr++;
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clockReg >>= 1;
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if (address & 0x01) {
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clockReg |= 0x8000000000000000LL;
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}
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if (clockRegPtr == 64) {
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regEnabled = false;
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// The clockReg should now contain a BCD4 packed date like
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// 0x1708071521140200
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// ... 2017, August 07, <day of week?>; 21:14:02.00
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// where that <day of week> is clearly suspect. Probably because 2017
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// was too far in the future when this driver was written...
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clockRegPtr = 0;
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updateClockFromRegister();
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}
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}
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}
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void NoSlotClock::writeNibble(uint8_t n)
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{
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for (uint8_t i=0; i<4; i++) {
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clockReg <<= 1;
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if (n & 0x08) {
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clockReg |= 1;
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}
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n <<= 1;
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}
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}
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