The 6845 Cathode Ray Tube Controller (CRTC)

Introduction

The 6845 Cathode Ray Tube Controller (CRTC) is a programmable IC used to generate video displays. This IC is used in a variety of computers including the Amstrad CPC, Amstrad CPC+ and KC Compact.

CRTC in the Amstrad CPC/CPC+ and KC Compact designs

The CRTC was a common part available from many different manufacturers. During the life of the CPC, Amstrad sourced the CRTC from various manufacturers.

All ICs used were based on the same design but have a different implementation. As a result they do not operate identically in all situations. This document highlights these differences.

This table lists the known ICs used, with their part number, manufacturer and type number.

Part number	Manufacturer	Type number (note 3)
UM6845	UMC	0
HD6845S	Hitachi	0
UM6845R	UMC	1
MC6845	Motorola	2
AMS40489	Amstrad	3 (note 1)
???	Amstrad?	4 (note 2)

NOTES:

1. The CRTC functionality is integrated into the CPC+ ASIC. This type exists only in the CPC464+,CPC6128+ and GX4000.
2. As far as I know, this type exists only in "cost-down" CPC6128 systems. In the "cost-down" CPC6128, the CRTC functionality is integrated into a single ASIC IC. This ASIC is often refered to as the "Pre-ASIC" because it preceeded the CPC+ ASIC.

3. In the Amstrad community each 6845 implementation has been assigned a type number. This type identifies a group of implementations which operate in exactly the same way.

   As far as I know, the type number system was originally used by demo programmers.

   It is possible to detect the 6845 present using software methods, and this is done to:

   • warn that the software was not designed for the detected 6845 and may function incorrectly,
   • to adapt the software so that it will run with the detected 6845

   In most cases, the type of the detected 6845 is reported.

4. As far as I know, the KC compact used HD6845S only.

The 6845 is selected when bit 14 of the I/O port address is set to "0". Bit 1 and 0 of the I/O port address define the function to access. The remaining bits can be any value, but it is adviseable to set these to "1" to avoid conflict with other devices in the system.

The recommended I/O port addressess are:

I/O port address	Function	Read/Write
&BCxx	Select 6845 register	Write only
&BDxx	Write 6845 register data	Write only
&BExx	(note 1)	Read only
&BFxx	(note 1)	Read only

NOTE:

1. The function of these I/O ports is dependant on the CRTC type

Signals

The following table defines the generated memory address from the CRTC and Gate-Array signals.

Memory Address Signal	Signal source	Signal name
A15	6845	MA13
A15	6845	MA12
A14	6845	MA11
A13	6845	RA2
A12	6845	RA1
A11	6845	RA0
A10	6845	MA9
A9	6845	MA8
A8	6845	MA7
A7	6845	MA6
A6	6845	MA5
A5	6845	MA4
A4	6845	MA3
A3	6845	MA2
A2	6845	MA1
A1	6845	MA0
A0	Gate-Array	CCLK

DISPTMG

DISPTMG signal defines the border. When DISPTMG is "1" the border colour is output to the display.

HSYNC and VSYNC

HSYNC and VSYNC from the CRTC are passed into the Gate-Array. The Gate-Array modifies the signals and then mixes these to form the Composite-Sync which is output to the display

The 6845 Design

Registers

The Internal registers of the 6845 are:

Register Index	Register Name
0	Horizontal Total
1	Horizontal Displayed
2	Horizontal Sync Position
3	Horizontal and Vertical Sync Widths
4	Vertical Total
5	Vertical Total Adjust
6	Vertical Displayed
7	Vertical Sync position
8	Interlace and Skew
9	Maximum Raster Address
10	Cursor Start Raster
11	Cursor End Raster
12	Display Start Address (High)
13	Display Start Address (Low)
14	Cursor Address (High)
15	Cursor Address (High)
16	Light Pen Address (High)
17	Light Pen Address (High)

CRTC Differences

In this section I will attempt to identify all the differences between each CRTC.

The following tables list the functions that can be accessed for each type:

Type 0

Type 1

Type 2

Type 3 and 4

It is not possible to read from all the internal registers, this table shows the read/write status of each register for each type:

Register Index	Register Name	Type
		0	1	2	3
0	Horizontal Total	Write Only	Write Only	Write Only	(note 2)
1	Horizontal Displayed	Write Only	Write Only	Write Only	(note 2)
2	Horizontal Sync Position	Write Only	Write Only	Write Only	(note 2)
3	Horizontal and Vertical Sync Widths	Write Only	Write Only	Write Only	(note 2)
4	Vertical Total	Write Only	Write Only	Write Only	(note 2)
5	Vertical Total Adjust	Write Only	Write Only	Write Only	(note 2)
6	Vertical Displayed	Write Only	Write Only	Write Only	(note 2)
7	Vertical Sync position	Write Only	Write Only	Write Only	(note 2)
8	Interlace and Skew	Write Only	Write Only	Write Only	(note 2)
9	Maximum Raster Address	Write Only	Write Only	Write Only	(note 2)
10	Cursor Start Raster	Write Only	Write Only	Write Only	(note 2)
11	Cursor End Raster	Write Only	Write Only	Write Only	(note 2)
12	Display Start Address (High)	Read/Write	Write Only	Write Only	Read/Write (note 2)
13	Display Start Address (Low)	Read/Write	Write Only	Write Only	Read/Write (note 2)
14	Cursor Address (High)	Read/Write	Read/Write	Read/Write	Read/Write (note 2) ***check
15	Cursor Address (Low)	Read/Write	Read/Write	Read/Write	Read/Write (note 2) **check
16	Light Pen Address (High)	Read Only	Read Only	Read Only	Read Only (note 2) **check
17	Light Pen Address (High)	Read Only	Read Only	Read Only	Read Only (note 2) **check

Notes:

1. On type 0 and 1, if a Write Only register is read from, "0" is returned.
2. See the document "Extra CPC Plus Hardware Information" for more details.