Converting Amstrad CPC Screen Dumps to PNG: CRTC 6845 Framebuffer Decoding

Throughout the 1980s and early 1990s, the Amstrad CPC 464, 664, and 6128 microcomputers were among the most popular home computers across Britain, France, Spain, and Germany. Known for their colorful games and vibrant demoscene, Amstrad computers featured a distinctive video display architecture driven by the Motorola 6845 Cathode Ray Tube Controller (CRTC) and the custom Amstrad Gate Array.

Art files, game title screens, and loading images were dumped from memory as 16KB binary buffers ending in .cpc or .scr.

Because the CPC's display hardware organizes pixels in a non-linear scanline layout and interleaves color bits within every byte, modern image software cannot directly display raw CPC files.

This technical guide reveals the memory addressing of the CRTC 6845, details the bitplane mapping of CPC graphics Modes 0, 1, and 2, and explains how convrtr's CPC to PNG converter renders retro Amstrad screens into crystal-clear 32-bit RGBA PNG images in your browser.

The 16KB Video RAM and AMSDOS Header

An authentic Amstrad CPC screen dump is exactly 16,384 bytes (16 KB), representing the entire video RAM typically mapped from 0xC000 to 0xFFFF.

When saved onto Amstrad AMSDOS floppy disks or cassette tapes, the file is prefixed by a 128-byte AMSDOS header (resulting in a 16,512-byte file):

• Byte 0: User number (typically 0).
• Bytes 1–8: Filename (ASCII padded with spaces).
• Bytes 9–11: Extension (e.g., BIN or SCR).
• Byte 18: File type (2 denotes a screen memory dump).
• Bytes 21–22: Load address (0xC000 in little-endian).
• Bytes 24–25: File length (0x4000 = 16,384 bytes).
• Bytes 67–68: 16-bit checksum of bytes 0 through 66.

convrtr verifies the AMSDOS header checksum, extracts the original retro filename, and jumps directly to the framebuffer memory offset.

The CRTC 6845 Non-Linear Scanline Addressing

Unlike modern graphics framebuffers where scanlines are contiguous arrays in memory, the Motorola 6845 addresses the screen as character rows divided into character scanline slices:

lineInCharacter = y % 8;
characterRow    = Math.floor(y / 8);
memoryOffset    = (lineInCharacter * 0x800) + (characterRow * 80);

Each horizontal line is 80 bytes wide. The 8 scanlines of a character row are spaced 2,048 bytes (0x800) apart in RAM, meaning that adjacent scanlines on your monitor reside in completely different 2KB memory blocks.

Graphics Modes and Bitplane Interleaving

The Amstrad CPC Gate Array generates three standard graphics modes from the 80-byte scanlines:

Mode 0: 160 × 200 (16 Colors)

Mode 0 produces colorful, chunky pixel art. Each byte stores 2 pixels (4 bits per pixel). However, the bits are interleaved across the byte:

Pixel 0 = Bit7 | (Bit3 << 1) | (Bit5 << 2) | (Bit1 << 3)
Pixel 1 = Bit6 | (Bit2 << 1) | (Bit4 << 2) | (Bit0 << 3)

Because Mode 0 pixels are rectangular (2:1 aspect ratio on a 4:3 CRT), convrtr applies automatic aspect correction by rendering each pixel 2 pixels wide (320 × 200 output).

Mode 1: 320 × 200 (4 Colors)

Mode 1 provides sharp 4-color graphics. Each byte stores 4 pixels (2 bits per pixel):

Pixel 0 = Bit7 | (Bit3 << 1)
Pixel 1 = Bit6 | (Bit2 << 1)
Pixel 2 = Bit5 | (Bit1 << 1)
Pixel 3 = Bit4 | (Bit0 << 1)

Mode 2: 640 × 200 (2 Colors)

Mode 2 is high-resolution monochrome. Each byte stores 8 single-bit pixels from MSB (left) to LSB (right).

The 27-Color Gate Array Palette

The Amstrad Gate Array produces analog RGB signals with 3 discrete intensity levels per channel: 0%, 50%, and 100%. This yields a unique 27-color palette featuring vibrant primaries and distinctive pastel shades.

convrtr maps each pen index to its corresponding Gate Array RGB color, outputting an authentic 32-bit RGBA PNG image with zero blur and zero compression artifacts.