Got main system and VDP working! There is a display!
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194
Core/Video/SmsVdp.cs
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194
Core/Video/SmsVdp.cs
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using System;
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namespace Core.Video
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{
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public class SmsVdp
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{
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// The VDP's private memory! The CPU cannot touch these arrays directly.
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public byte[] VRAM { get; private set; } = new byte[0x4000]; // 16KB Video RAM
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public byte[] CRAM { get; private set; } = new byte[0x20]; // 32 Bytes Color Palette
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public byte[] Registers { get; private set; } = new byte[16]; // 11 Hardware Control Registers
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public int[] FrameBuffer { get; private set; } = new int[256 * 192];
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// The Control Port State Machine (Port 0xBF)
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private bool _isSecondControlByte = false;
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private ushort _controlWord = 0;
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private byte _readBuffer = 0;
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private int _tStateCounter = 0;
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private int _currentScanline = 0;
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private byte _statusRegister = 0x00;
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public bool InterruptPending => (_statusRegister & 0x80) != 0 && (Registers[1] & 0x20) != 0;
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public byte ReadDataPort() // Port 0xBE
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{
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_isSecondControlByte = false; // Reading data resets the control latch
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byte value = _readBuffer;
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_readBuffer = VRAM[_controlWord & 0x3FFF];
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_controlWord++;
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return value;
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}
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public byte ReadControlPort() // Port 0xBF
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{
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_isSecondControlByte = false;
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byte currentStatus = _statusRegister;
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// CRITICAL HARDWARE QUIRK: Reading the status port physically
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// clears the flags inside the chip! If we don't clear this,
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// the interrupt line gets stuck on forever.
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_statusRegister = 0x00;
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return currentStatus;
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}
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public void WriteDataPort(byte value) // Port 0xBE
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{
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_isSecondControlByte = false;
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_readBuffer = value;
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int address = _controlWord & 0x3FFF;
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int command = (_controlWord >> 14) & 0x03;
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if (command == 3) // Code 3: Write to Color Palette (CRAM)
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{
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CRAM[address & 0x1F] = value;
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}
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else // Code 0, 1, 2: Write to VRAM
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{
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VRAM[address] = value;
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}
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_controlWord++; // Auto-increment so the Z80 can blast data fast
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}
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public void WriteControlPort(byte value) // Port 0xBF
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{
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if (!_isSecondControlByte)
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{
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// First byte arrives: Store it in the lower 8 bits
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_controlWord = (ushort)((_controlWord & 0xFF00) | value);
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_isSecondControlByte = true;
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}
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else
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{
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// Second byte arrives: Store it in the upper 8 bits and execute!
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_controlWord = (ushort)((_controlWord & 0x00FF) | (value << 8));
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_isSecondControlByte = false;
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int command = (_controlWord >> 14) & 0x03;
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if (command == 0) // Code 0: Prep for VRAM Read
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{
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_readBuffer = VRAM[_controlWord & 0x3FFF];
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_controlWord++;
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}
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else if (command == 2) // Code 2: Write to Internal VDP Register
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{
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int regIndex = value & 0x0F;
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byte regData = (byte)(_controlWord & 0xFF);
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if (regIndex < 16) Registers[regIndex] = regData;
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}
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}
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}
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public byte ReadVCounter()
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{
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// Note: On real NTSC hardware, the V-Counter jumps slightly around
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// the VBlank period to keep the math 8-bit, but simply returning
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// the raw current scanline is perfectly fine to get us booting!
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return (byte)_currentScanline;
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}
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public void Update(int tStates)
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{
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_tStateCounter += tStates;
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// 228 T-States per scanline
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if (_tStateCounter >= 228)
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{
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_tStateCounter -= 228;
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_currentScanline++;
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// Line 192 is the exact moment the screen finishes drawing!
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if (_currentScanline == 192)
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{
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_statusRegister |= 0x80; // Set Bit 7 (VBlank Flag) to 1
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RenderBackground(); // <--- DRAW THE FRAME!
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}
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// End of the NTSC frame (262 lines)
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if (_currentScanline > 261)
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{
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_currentScanline = 0;
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}
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}
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}
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private void RenderBackground()
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{
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// The Name Table base address is stored in VDP Register 2.
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// It tells us where in VRAM the 32x24 screen grid starts.
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ushort nameTableBase = (ushort)((Registers[2] & 0x0E) << 10);
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// Loop through all 24 rows and 32 columns of the screen
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for (int row = 0; row < 24; row++)
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{
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for (int col = 0; col < 32; col++)
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{
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// 1. Read the 16-bit Tile instruction from the Name Table
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ushort nameTableAddr = (ushort)(nameTableBase + (row * 64) + (col * 2));
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byte lowByte = VRAM[nameTableAddr];
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byte highByte = VRAM[nameTableAddr + 1];
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ushort tileData = (ushort)((highByte << 8) | lowByte);
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// 2. Extract the Tile Index and Palette Info
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int tileIndex = tileData & 0x01FF;
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bool useSpritePalette = (tileData & 0x0800) != 0;
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// 3. Find the actual pixel data for this tile in VRAM
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// Each 8x8 tile takes exactly 32 bytes in memory
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ushort tileAddress = (ushort)(tileIndex * 32);
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// 4. Draw the 8x8 block of pixels!
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for (int y = 0; y < 8; y++)
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{
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// The SMS uses 4 bitplanes to make a single row of pixels.
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byte bp0 = VRAM[tileAddress + (y * 4) + 0];
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byte bp1 = VRAM[tileAddress + (y * 4) + 1];
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byte bp2 = VRAM[tileAddress + (y * 4) + 2];
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byte bp3 = VRAM[tileAddress + (y * 4) + 3];
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for (int x = 0; x < 8; x++)
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{
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// Combine 1 bit from each bitplane to get a color index (0-15)
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int shift = 7 - x;
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int colorIndex = ((bp0 >> shift) & 1) |
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(((bp1 >> shift) & 1) << 1) |
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(((bp2 >> shift) & 1) << 2) |
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(((bp3 >> shift) & 1) << 3);
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// Find the raw SMS color in CRAM
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int paletteOffset = useSpritePalette ? 16 : 0;
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byte smsColor = CRAM[paletteOffset + colorIndex];
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// Translate SMS 00BBGGRR format to Windows 32-bit ARGB
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int r = (smsColor & 0x03) * 85;
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int g = ((smsColor >> 2) & 0x03) * 85;
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int b = ((smsColor >> 4) & 0x03) * 85;
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// Calculate where this pixel goes on the final 256x192 screen
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int pixelX = (col * 8) + x;
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int pixelY = (row * 8) + y;
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FrameBuffer[(pixelY * 256) + pixelX] = (255 << 24) | (r << 16) | (g << 8) | b;
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}
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}
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}
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}
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}
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}
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}
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