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240 lines
7.4 KiB
C++
240 lines
7.4 KiB
C++
//
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// ClockConverter.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 01/01/2023.
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// Copyright © 2023 Thomas Harte. All rights reserved.
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//
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#ifndef ClockConverter_hpp
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#define ClockConverter_hpp
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#include "../9918.hpp"
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#include "PersonalityTraits.hpp"
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namespace TI {
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namespace TMS {
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// Timing constants.
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template <Personality, typename Enable = void> struct Timing {};
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template <Personality personality>
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struct Timing<personality, std::enable_if_t<is_classic_vdp(personality)>> {
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/// The total number of internal cycles per line of output.
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constexpr static int CyclesPerLine = 342;
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/// The number of internal cycles that must elapse between a request to read or write and
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/// it becoming a candidate for action.
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constexpr static int VRAMAccessDelay = 6;
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/// The first internal cycle at which pixels will be output in any mode other than text.
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/// Pixels implicitly run from here to the end of the line.
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constexpr static int FirstPixelCycle = 86;
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/// Indicates whether text modes are supported by this VDP. If so then
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/// values for First & Last TextCycle are required. If not then they can be omitted.
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constexpr static bool SupportsTextMode = true;
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/// The first internal cycle at which pixels will be output text mode.
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constexpr static int FirstTextCycle = 94;
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/// The final internal cycle at which pixels will be output text mode.
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constexpr static int LastTextCycle = 334;
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// For the below, the fixed portion of line layout is:
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//
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// [0, EndOfRightBorder): right border colour
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// [EndOfRightBorder, StartOfSync): blank
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// [StartOfSync, EndOfSync): sync
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// [EndOfSync, StartOfColourBurst): blank
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// [StartOfColourBurst, EndOfColourBurst): the colour burst
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// [EndOfColourBurst, StartOfLeftBorder): blank
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//
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// The region from StartOfLeftBorder until the end is then filled with
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// some combination of pixels and more border, depending on the vertical
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// position of this line and the current screen mode.
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constexpr static int EndOfRightBorder = 15;
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constexpr static int StartOfSync = 23;
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constexpr static int EndOfSync = 49;
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constexpr static int StartOfColourBurst = 51;
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constexpr static int EndOfColourBurst = 65;
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constexpr static int StartOfLeftBorder = 73;
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};
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template <Personality personality>
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struct Timing<personality, std::enable_if_t<is_yamaha_vdp(personality)>> {
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constexpr static int CyclesPerLine = 1368;
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constexpr static int VRAMAccessDelay = 6;
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constexpr static int FirstPixelCycle = 344;
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constexpr static bool SupportsTextMode = true;
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constexpr static int FirstTextCycle = 376;
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constexpr static int LastTextCycle = 1336;
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constexpr static int EndOfRightBorder = 15 * 4;
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constexpr static int StartOfSync = 23 * 4;
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constexpr static int EndOfSync = 49 * 4;
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constexpr static int StartOfColourBurst = 51 * 4;
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constexpr static int EndOfColourBurst = 65 * 4;
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constexpr static int StartOfLeftBorder = 73 * 4;
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};
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template <>
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struct Timing<Personality::MDVDP> {
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constexpr static int CyclesPerLine = 3420;
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constexpr static int VRAMAccessDelay = 6;
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constexpr static int FirstPixelCycle = 860;
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constexpr static bool SupportsTextMode = false;
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// Implementation note: these currently need to be multiples of 2.5
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// per the stateless Mega Drive -> CRT clock conversion.
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constexpr static int EndOfRightBorder = 15 * 10;
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constexpr static int StartOfSync = 23 * 10;
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constexpr static int EndOfSync = 49 * 10;
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constexpr static int StartOfColourBurst = 51 * 10;
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constexpr static int EndOfColourBurst = 65 * 10;
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constexpr static int StartOfLeftBorder = 73 * 10;
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};
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constexpr int TMSAccessWindowsPerLine = 171;
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/*!
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This implementation of the TMS, etc mediates between three clocks:
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1) the external clock, which is whatever the rest of the system(s)
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it plugs into run at;
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2) the internal clock, which is used to time and place syncs, borders,
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pixel regions, etc; and
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3) a memory acccess clock, which correlates to the number of windows
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available for memory accesses.
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E.g. for both a regular TMS9918 and the Sega Master System, the external
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clock is 3.58Mhz, the internal clock is 5.37Mhz and the memory access
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clock is 2.69Mhz.
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Or, put another way, for both a TMS9918 and Master System:
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* 228 external cycles;
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* is 342 internal cycles;
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* which exactly covers 228 NTSC colour clocks; and
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* contains 171 memory access windows.
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Both the Yamaha extensions and the Mega Drive VDP are a bit smarter about
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paged mode memory accesses, obviating any advantage to treating (3) as a
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separate clock.
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*/
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template <Personality personality> class ClockConverter {
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public:
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/*!
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Given that another @c source external **half-cycles** has occurred,
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indicates how many complete internal **cycles** have additionally elapsed
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since the last call to @c to_internal.
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E.g. for the TMS, @c source will count 456 ticks per line, and the internal clock
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runs at 342 ticks per line, so the proper conversion is to multiply by 3/4.
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*/
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int to_internal(int source) {
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switch(personality) {
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// Default behaviour is to apply a multiplication by 3/4;
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// this is correct for the TMS and Sega VDPs other than the Mega Drive.
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default: {
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const int result = source * 3 + cycles_error_;
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cycles_error_ = result & 3;
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return result >> 2;
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}
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// The two Yamaha chips have an internal clock that is four times
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// as fast as the TMS, therefore a stateless translation is possible.
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case Personality::V9938:
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case Personality::V9958:
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return source * 3;
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// The Mega Drive runs at 3420 master clocks per line, which is then
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// divided by 4 or 5 depending on other state. That's 7 times the
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// rate provided to the CPU; given that the input is in half-cycles
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// the proper multiplier is therefore 3.5.
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case Personality::MDVDP: {
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const int result = source * 7 + cycles_error_;
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cycles_error_ = result & 1;
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return result >> 1;
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}
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}
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}
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/*!
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Provides the number of complete external cycles that lie between now and
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@c internal_cycles into the future. Any trailing fractional external cycle
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is discarded.
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*/
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HalfCycles half_cycles_before_internal_cycles(int internal_cycles) const {
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// Logic here correlates with multipliers as per @c to_internal.
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switch(personality) {
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default:
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return HalfCycles(
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((internal_cycles << 2) + (2 - cycles_error_)) / 3
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);
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case Personality::V9938:
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case Personality::V9958:
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return HalfCycles(internal_cycles / 3);
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case Personality::MDVDP:
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return HalfCycles(
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((internal_cycles << 1) + (1 - cycles_error_)) / 7
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);
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}
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}
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/*!
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Converts a position in internal cycles to its corresponding position
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on the TMS memory-access clock, i.e. scales down to 171 clocks
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per line
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*/
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static constexpr int to_tms_access_clock(int source) {
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switch(personality) {
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default:
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return source >> 1;
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case Personality::V9938:
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case Personality::V9958:
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return source >> 3;
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case Personality::MDVDP:
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return source / 20;
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}
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}
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/*!
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Convers a position in internal cycles to its corresponding position
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on the CRT's output clock, which [TODO] is clocked so that
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1368 cycles is 228 NTSC colour cycles.
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*/
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static constexpr int to_crt_clock(int source) {
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switch(personality) {
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default:
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return source * 4;
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case Personality::V9938:
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case Personality::V9958:
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return source;
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case Personality::MDVDP:
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return (source * 2) / 5;
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}
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}
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private:
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// Holds current residue in conversion from the external to
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// internal clock.
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int cycles_error_ = 0;
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};
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}
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}
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#endif /* ClockConverter_hpp */
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