Program VGA Registers for Graphics Modes

Topic Overview

VGA register programming

Beyond BIOS, direct VGA access uses attribute controller, sequencer, and graphics controller ports unlocked via the feature control register. Mode 13h is easy; high-resolution modes need register sets from VESA or custom tables. For example, writing sequencer index 0x00 to port 0x3C4 begins a register sequence that sets dot clock and memory mode.

Port map basics

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• DAC: Port 0x3C8/0x3C9 for palette in 256-color modes
• Plane manipulation: Graphics controller maps writes to planes 0-3
• Chain-4: Sequencer memory mode affects how bytes hit video RAM
• Mistimed register writes produce snow or blank screens

Mode 13h vs planar

Mode 13h is popular for demos because it is linear-enough for byte writes, while 640x480x16 requires four plane passes per pixel. Sequencer memory mode and chain-4 settings determine whether a byte hits one plane or all planes. Always unlock the attribute controller by reading 0x3DA (input status) before writing indexed registers. Snow artifacts mean the CRTC was reprogrammed during active display time.

Why for this exercise

You will switch or tweak a VGA mode using register I/O instead of INT 0x10. This exercise requires documenting index/data port pairs and a concrete register write sequence for one mode.

Compute VGA register values in C from stdin requests (80x25 text mode and mode 13h).

Input format:

• Line 1: N, the number of commands.
• Then one command per line:
  • "CURSOR <col> <row>": hardware text cursor cell (col 0-79, row 0-24)
  • "PUTC <col> <row>": cell whose character+attribute pair is written
  • "PIXEL <x> <y>": mode 13h pixel (x 0-319, y 0-199)
  • "DAC <idx> <r> <g> <b>": palette entry (idx 0-255), 8-bit channels

Requirements:

• CURSOR: pos = row*80 + col; CRTC registers 0x0E (high) and 0x0F (low) receive pos>>8 and pos&0xFF via ports 0x3D4/0x3D5. Print "CURSOR <col> <row> -> pos=<p> hi=0x<hh> lo=0x<hh> (idx 0x0E/0x0F)".
• PUTC: the cell at pos = row80 + col lives at offset 2pos in the 0xB8000 text buffer. Print "PUTC <col> <row> -> addr=0x<ppppp> off=0x<hhhh>".
• PIXEL: mode 13h offset = y*320 + x in the linear 0xA0000 framebuffer. Print "PIXEL <x> <y> -> addr=0x<ppppp> off=0x<hhhh>".
• DAC: the DAC stores 6-bit channels, so each 8-bit value is written as value>>2 (0-63). Print "DAC <idx> -> rgb=63 <r6> <g6> <b6>".
• Out-of-range col/row/x/y/idx prints the command with "-> error <bounds text>" instead of values; never compute past bounds.
• Exit code 0.