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148 lines
3.9 KiB
C++
148 lines
3.9 KiB
C++
//
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// Copper.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 16/09/2021.
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// Copyright © 2021 Thomas Harte. All rights reserved.
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//
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#ifndef NDEBUG
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#define NDEBUG
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#endif
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#define LOG_PREFIX "[Copper] "
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#include "../../Outputs/Log.hpp"
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#include "Chipset.hpp"
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#include "Copper.hpp"
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using namespace Amiga;
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namespace {
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bool satisfies_raster(uint16_t position, uint16_t blitter_status, uint16_t *instruction) {
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// Return immediately if: (i) wait-for-Blitter is not disabled; and (ii) the Blitter is busy.
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if(!(instruction[1] & 0x8000) && (blitter_status & 0x4000)) {
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return false;
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}
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// Otherwise, test the raster position against the instruction's value and mask.
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const uint16_t mask = 0x8000 | (instruction[1] & 0x7ffe);
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return (position & mask) >= (instruction[0] & mask);
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}
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}
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//
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// Quick notes on the Copper:
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//
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// There are three instructions: move, wait and skip. All are two words in length.
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//
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// Move writes a value to one of the Chipset registers; it is encoded as:
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//
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// First word:
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// b0: 0
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// b1–b8: register address
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// b9+: unused ("should be set to 0")
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//
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// Second word:
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// b0–b15: value to move.
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//
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//
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// Wait waits until the raster gets to at least a certain position, and
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// optionally until the Blitter has finished. It is encoded as:
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//
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// First word:
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// b0: 1
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// b1–b7: horizontal beam position
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// b8+: vertical beam position
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//
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// Second word:
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// b0: 0
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// b1–b7: horizontal beam comparison mask
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// b8–b14: vertical beam comparison mask
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// b15: 1 => don't also wait for the Blitter to be finished; 0 => wait.
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//
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//
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// Skip skips the next instruction if the raster has already reached a certain
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// position, and optionally only if the Blitter has finished, and only if the
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// next instruction is a move.
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//
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// First word:
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// b0: 1
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// b1–b7: horizontal beam position
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// b8+: vertical beam position
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//
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// Second word:
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// b0: 1
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// b1–b7: horizontal beam comparison mask
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// b8–b14: vertical beam comparison mask
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// b15: 1 => don't also test whether the Blitter is finished; 0 => test.
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//
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bool Copper::advance_dma(uint16_t position, uint16_t blitter_status) {
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switch(state_) {
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default: return false;
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case State::Waiting:
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if(satisfies_raster(position, blitter_status, instruction_)) {
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LOG("Unblocked waiting for " << PADHEX(4) << instruction_[0] << " at " << PADHEX(4) << position << " with mask " << PADHEX(4) << (instruction_[1] & 0x7ffe));
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state_ = State::FetchFirstWord;
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}
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return false;
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case State::FetchFirstWord:
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instruction_[0] = ram_[address_ & ram_mask_];
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++address_;
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state_ = State::FetchSecondWord;
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LOG("First word fetch at " << PADHEX(4) << position);
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break;
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case State::FetchSecondWord: {
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// Get and reset the should-skip-next flag.
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const bool should_skip_move = skip_next_;
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skip_next_ = false;
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// Read in the second instruction word.
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instruction_[1] = ram_[address_ & ram_mask_];
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++address_;
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LOG("Second word fetch at " << PADHEX(4) << position);
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// Check for a MOVE.
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if(!(instruction_[0] & 1)) {
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if(!should_skip_move) {
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// Stop if this move would be a privilege violation.
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instruction_[0] &= 0x1fe;
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if((instruction_[0] < 0x10) || (instruction_[0] < 0x20 && !(control_&1))) {
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LOG("Invalid MOVE to " << PADHEX(4) << instruction_[0] << "; stopping");
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state_ = State::Stopped;
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break;
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}
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chipset_.write(instruction_[0], instruction_[1]);
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}
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// Roll onto the next command.
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state_ = State::FetchFirstWord;
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break;
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}
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// Got to here => this is a WAIT or a SKIP.
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if(!(instruction_[1] & 1)) {
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// A WAIT. The wait-for-start-of-next PAL wait of
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// $FFDF,$FFFE seems to suggest evaluation will happen
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// in the next cycle rather than this one.
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state_ = State::Waiting;
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break;
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}
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// Neither a WAIT nor a MOVE => a SKIP.
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skip_next_ = satisfies_raster(position, blitter_status, instruction_);
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state_ = State::FetchFirstWord;
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} break;
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}
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return true;
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}
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