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126 lines
3.7 KiB
C++
126 lines
3.7 KiB
C++
//
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// Sprites.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 26/11/2021.
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// Copyright © 2021 Thomas Harte. All rights reserved.
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//
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#include "Sprites.hpp"
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#include <cassert>
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using namespace Amiga;
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namespace {
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/// Expands @c source from b15 ... b0 to 000b15 ... 000b0.
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constexpr uint64_t expand_sprite_word(uint16_t source) {
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uint64_t result = source;
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result = (result | (result << 24)) & 0x0000'00ff'0000'00ff;
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result = (result | (result << 12)) & 0x000f'000f'000f'000f;
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result = (result | (result << 6)) & 0x0303'0303'0303'0303;
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result = (result | (result << 3)) & 0x1111'1111'1111'1111;
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return result;
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}
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// A very small selection of test cases.
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static_assert(expand_sprite_word(0xffff) == 0x11'11'11'11'11'11'11'11);
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static_assert(expand_sprite_word(0x5555) == 0x01'01'01'01'01'01'01'01);
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static_assert(expand_sprite_word(0xaaaa) == 0x10'10'10'10'10'10'10'10);
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static_assert(expand_sprite_word(0x0000) == 0x00'00'00'00'00'00'00'00);
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}
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// MARK: - Sprites.
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void Sprite::set_start_position(uint16_t value) {
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// b8–b15: low 8 bits of VSTART;
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// b0–b7: high 8 bits of HSTART.
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v_start_ = (v_start_ & 0xff00) | (value >> 8);
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h_start = uint16_t((h_start & 0x0001) | ((value & 0xff) << 1));
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}
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void Sprite::set_stop_and_control(uint16_t value) {
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// b8–b15: low 8 bits of VSTOP;
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// b7: attachment flag;
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// b3–b6: unused;
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// b2: VSTART high bit;
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// b1: VSTOP high bit;
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// b0: HSTART low bit.
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h_start = uint16_t((h_start & 0x01fe) | (value & 0x01));
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v_stop_ = uint16_t((value >> 8) | ((value & 0x02) << 7));
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v_start_ = uint16_t((v_start_ & 0x00ff) | ((value & 0x04) << 6));
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attached = value & 0x80;
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// Disarm the sprite.
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visible = false;
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}
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void Sprite::set_image_data(int slot, uint16_t value) {
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// Store data; also mark sprite as visible (i.e. 'arm' it)
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// if data is being stored to slot 0.
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data[slot] = value;
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visible |= slot == 0;
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}
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bool Sprite::advance_dma(int offset, int y, bool is_first_line) {
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assert(offset == 0 || offset == 1);
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// Determine which word would be fetched, if DMA occurs.
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// A bit of a cheat.
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const uint16_t next_word = ram_[pointer_[0] & ram_mask_];
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// "When the vertical position of the beam counter is equal to the VSTOP
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// value in the sprite control words, the next two words fetched from the
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// sprite data structure are written into the sprite control registers
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// instead of being sent to the color registers"
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//
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// Guesswork, primarily from observing Spindizzy Worlds: the first line after
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// vertical blank also triggers a control reload. Seek to verify.
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if(y == v_stop_ || is_first_line) {
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if(offset) {
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// Second control word: stop position (mostly).
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set_stop_and_control(next_word);
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} else {
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// First control word: start position.
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set_start_position(next_word);
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}
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} else {
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visible |= y == v_start_;
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if(!visible) return false; // Act as if there wasn't a fetch.
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// Write colour word 1, then colour word 0; 0 is the word that 'arms'
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// the sprite (i.e. makes it visible).
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set_image_data(1 - offset, next_word);
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}
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// Acknowledge the fetch.
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++pointer_[0];
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return true;
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}
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template <int sprite> void TwoSpriteShifter::load(
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uint16_t lsb,
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uint16_t msb,
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int delay) {
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constexpr int sprite_shift = sprite << 1;
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const int delay_shift = delay << 2;
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// Clear out any current sprite pixels; this is a reload.
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data_ &= 0xcccc'cccc'cccc'ccccull >> (sprite_shift + delay_shift);
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// Map LSB and MSB up to 64-bits and load into the shifter.
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const uint64_t new_data =
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(
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expand_sprite_word(lsb) |
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(expand_sprite_word(msb) << 1)
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) << sprite_shift;
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data_ |= new_data >> delay_shift;
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overflow_ |= uint8_t((new_data << 8) >> delay_shift);
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}
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template void TwoSpriteShifter::load<0>(uint16_t, uint16_t, int);
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template void TwoSpriteShifter::load<1>(uint16_t, uint16_t, int);
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