crtc-expert

Development

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.

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How to use this skill

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I want to install this Agent Skill for this project in Codex.

Source SKILL.md: https://github.com/majiayu000/claude-skill-registry/blob/HEAD/skills/data/crtc-expert/SKILL.md

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Copying this prompt does not install or run the skill. Review third-party files before use. Codex skill guide

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