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Agent skill
crtc-expert
Expert on MC6845 CRTC for ES-1841 video timing. Provides guidance on video timing, cursor control, display addressing, V-Blank detection, and CGA register programming.
Install this agent skill to your Project
npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/crtc-expert
SKILL.md
CRTC Expert - MC6845 / KR1810VG85
Expert knowledge for the ES-1841's CRT controller.
Key Specifications
| Property | Value |
|---|---|
| Soviet Clone | KR1810VG85 |
| I/O Ports | 3D4h/3D5h (index/data) |
| Registers | R0-R17 |
| Video RAM | B8000h |
I/O Ports
| Port | Function |
|---|---|
3D4h |
Index register (W) |
3D5h |
Data register (R/W) |
3D8h |
Mode Control (W) |
3D9h |
Color Select (W) |
3DAh |
Status Register (R) |
Key Registers
| Reg | Name | 80×25 Value |
|---|---|---|
| R0 | Horizontal Total | 71h |
| R1 | Horizontal Displayed | 50h |
| R4 | Vertical Total | 1Fh |
| R6 | Vertical Displayed | 19h |
| R9 | Max Scan Line | 07h |
| R12-13 | Start Address | Display offset |
| R14-15 | Cursor Address | Cursor pos |
Status Register (3DAh)
| Bit | Function |
|---|---|
| 0 | Display enable (1=retrace) |
| 3 | Vertical Sync (1=V-Blank) |
CRITICAL: ES-1841 BIOS hangs if V-Blank (bit 3) doesn't toggle correctly!
V-Blank Timing
V-Blank Start = R6 × (R9 + 1)
V-Blank End = (R4 + 1) × (R9 + 1)
Status must update on every read.
Mode Control (3D8h)
| Bit | Function |
|---|---|
| 0 | 80-column (1) / 40-col (0) |
| 1 | Graphics (1) / Text (0) |
| 3 | Video enable |
| 4 | Hi-res 640×200 |
| 5 | Blink enable |
References
- references/implementation-guide.md - Complete CRTC/CGA implementation guide with code examples
- See references/ for detailed hardware documentation
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