mirror of
https://github.com/MoleskiCoder/EightBit.git
synced 2024-06-10 19:29:30 +00:00
140 lines
2.9 KiB
C++
140 lines
2.9 KiB
C++
#include "stdafx.h"
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#include "M6532.h"
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#include <cassert>
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EightBit::M6532::M6532() noexcept {
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}
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/*
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RAM—128 Bytes (1024 Bits)
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The 128 x 8 Read/Write memory acts as a conventional static RAM. Data can be written into the RAM from
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the microprocessor by selecting the chip (CS1 = 1, CS2 = 0) and by setting RS to a logic 0 (0.4v). Address
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lines AO through A6 are then used to select the desired byte of storage.
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*/
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/*
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__
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RS R/W A4 A3 A2 A1 A0
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Write RAM 0 0 - - - - -
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Read RAM 0 1 - - - - -
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Write DDRA 1 0 - - 0 0 1
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Read DDRA 1 1 - - 0 0 1
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Write DDRB 1 0 - - 0 1 1
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Read DDRB 1 1 - - 0 1 1
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Write Output Reg A 1 0 - - 0 0 0
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Read Output Reg A 1 1 - - 0 0 0
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Write Output Reg B 1 0 - - 0 1 0
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Read Output Reg B
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Write Timer
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1 1 0 1 0
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+ 1T 1 0 1 (a) 1 0 0
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+ 8T 1 0 1 (a) 1 0 1
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+ 64T t 0 1 (a) 1 1 0
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+ 1024T 1 0 1 (a) 1 1 1
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Read Timer 1 1 _ (a) 1 — 0
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Read Interrupt Flag(s) 1 1 — — 1 — 1
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Write Edge Detect Control 1 0 0 — 1 (b) (0)
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*/
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void EightBit::M6532::tick() {
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if (selected()) {
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// Process interrupts
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if (--m_currentIncrement == 0) {
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m_currentIncrement = m_timerIncrement;
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--m_timerInterval;
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}
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if (m_allowPA7Interrupts && (PA() & 0x80))
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IF() &= 0x40;
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if (m_allowTimerInterrupts && (m_timerInterval == 0))
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IF() &= 0x80;
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const auto read = raised(RW());
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const auto write = lowered(RW());
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assert(read == !write);
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const auto ram = lowered(RS());
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if (ram) {
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auto& cell = RAM().reference(address() & 0x7f);
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read ? data() = cell : cell = data();
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} else {
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const auto a0 = address() & 0b00001;
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const auto a1 = address() & 0b00010;
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const auto a2 = address() & 0b00100;
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const auto a3 = address() & 0b01000;
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const auto a4 = address() & 0b10000;
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const auto portControls = a2 == 0;
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const auto otherControls = a2 == 1;
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if (portControls) {
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switch (a0 | a1) {
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case 0b00:
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// R/W output reg A
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break;
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case 0b01:
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read ? data() = DDRA() : DDRA() = data();
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break;
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case 0b10:
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// R/W output reg B
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break;
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case 0b11:
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read ? data() = DDRB() : DDRB() = data();
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break;
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}
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} else {
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if (read && !a4 && a2) {
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m_allowPA7Interrupts = !a1;
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m_edgeDetection = a0 ? Positive : Negative;
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}
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if (read && a2 && a0)
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data() = IF() & (0x80 & 0x40);
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m_allowTimerInterrupts = !!a3;
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if (write && a4) {
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m_timerInterval = data();
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switch (a1 | a0) {
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case 0b00:
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m_timerIncrement = One;
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break;
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case 0b01:
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m_timerIncrement = Eight;
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break;
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case 0b10:
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m_timerIncrement = SixtyFour;
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break;
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case 0b11:
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m_timerIncrement = OneThousandAndTwentyFour;
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break;
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}
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m_currentIncrement = m_timerIncrement;
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}
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}
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}
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}
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}
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void EightBit::M6532::initialise() {
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}
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void EightBit::M6532::reset() {
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PA() = m_dra = m_ddra = 0; // Zero port A registers
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PB() = m_drb = m_ddrb = 0; // Zero port B registers
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m_allowTimerInterrupts = m_allowPA7Interrupts = false; // Interrupts are disabled
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}
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