#include <crtc.h>

Public Types
enum	Register : tUBYTE { HorizontalTotal = 0, HorizontalDisplayed = 1, HorizontalSyncPosition = 2, SyncWidth = 3, VerticalTotal = 4, VerticalTotalAdjust = 5, VerticalDisplayed = 6, VerticalSyncPosition = 7, InterlaceAndSkew = 8, MaximumRasterAddress = 9, CursorStartRaster = 10, CursorEndRaster = 11, StartAddressHigh = 12, StartAddressLow = 13, CursorHigh = 14, CursorLow = 15, LightPenHigh = 16, LightPenLow = 17 }

enum	Flags : tUWORD { VS = 1, HS = 2, HDT = 4, VDT = 8, HT = 16, VT = 32, MR = 64, VTadj = 128, VSf = 256 }

Public Member Functions
	Crtc ()

	~Crtc () ROLAND_DEFAULT void init(Ppi *ppi

tUBYTE	selected () const

tUBYTE	read () const

tUBYTE	read (tUBYTE num) const

void	select (tUBYTE reg)

void	write (tUBYTE reg, tUBYTE value)

void	write (tUBYTE value)

uint	flags ()

tUBYTE	hsw ()

tBYTE	hswVdu ()

tUBYTE	hswActive ()

tUBYTE	hswCount ()

tUBYTE	vsw ()

tUBYTE	vswCount ()

tUBYTE	lastHDisp ()

tUBYTE	skew ()

tUBYTE	vtAdjust ()

tUBYTE	vtAdjustCount ()

tUBYTE	maxRaster ()

tUBYTE	rasterCount ()

tUBYTE	lineCount ()

tUBYTE	charCount ()

uint	requestedAddr ()

uint	addr ()

void	setFlags (uint flags)

void	addFlags (uint flags)

void	removeFlags (uint flags)

void	setHswVdu (tUBYTE value)

void	setHswActive (tUBYTE value)

void	setHswCount (tUBYTE value)

void	setAddr (uint value)

void	setVtAdjust (tUBYTE value)

void	setVtAdjustCount (tUBYTE value)

void	setVswCount (tUBYTE value)

void	setLastHDisp (tUBYTE value)

void	setCharCount (tUBYTE value)

void	setLineCount (tUBYTE value)

void	setRasterCount (tUBYTE value)

void	setSkew (tUBYTE value)

void	incVswCount ()

Private Attributes
Ppi *	mPpi

tUBYTE	mRegister [18]

tUBYTE	mSelected

uint	mFlags

tUBYTE	mHsw

tBYTE	mHswVdu

tUBYTE	mHswCount

tUBYTE	mHswActive

tUBYTE	mVsw

tUBYTE	mVswCount

tUBYTE	mLastHDisp

tUBYTE	mSkew

tUBYTE	mVtAdjust

tUBYTE	mVtAdjustCount

tUBYTE	mMaxRaster

tUBYTE	mRasterCount

tUBYTE	mLineCount

tUBYTE	mCharCount

uint	mRequestedAddr

uint	mAddr

Detailed Description

The CRTC6845 CRTC6845

The 6845 Cathode Ray Tube Controller (CRTC)

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:

The CRTC functionality is integrated into the CPC+ ASIC. This type exists only in the CPC464+,CPC6128+ and GX4000.
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.
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.
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:

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

b1	b0	Function	Read/Write
0	0	Select internal 6845 register	Write Only
0	1	Write to selected internal 6845 register	Write Only
1	0	-	-
1	1	Read from selected internal 6845 register	Read only

Type 1

b1	b0	Function	Read/Write
0	0	Select internal 6845 register	Write Only
0	1	Write to selected internal 6845 register	Write Only
1	0	Read Status Register	Read Only
1	1	Read from selected internal 6845 register	Read only

Type 2

b1	b0	Function	Read/Write
0	0	Select internal 6845 register	Write Only
0	1	Write to selected internal 6845 register	Write Only
1	0	-	-
1	1	Read from selected internal 6845 register	Read only

Type 3 and 4

b1	b0	Function	Read/Write
0	0	Select internal 6845 register	Write Only
0	1	Write to selected internal 6845 register	Write Only
1	0	Read from selected internal 6845 register	Read Only
1	1	Read from selected internal 6845 register	Read only

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
Register Index	Register Name	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:

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

Member Enumeration Documentation

enum Flags : tUWORD

Enumerator
VS	VSync.
HS	HSync.
HDT	HorizontalDisplayedTotal.
VDT	VerticalDisplayedTotal.
HT	HorizontalTotal.
VT	VerticalTotal.
MR	MaximumRasterAddress.
VTadj	VerticalTotalAdjust.
VSf	VerticalSyncPosition.

enum Register : tUBYTE

Enumerator
HorizontalTotal	PAL SECAM NTSC. 63 63 63
HorizontalDisplayed	40 40 40
HorizontalSyncPosition	46 46 46
SyncWidth	0x8E 0x8E 0x8E
VerticalTotal	38 38 31
VerticalTotalAdjust	0 0 6
VerticalDisplayed	25 25 25
VerticalSyncPosition	30 30 27
InterlaceAndSkew	0 0 0
MaximumRasterAddress	7 7 7
CursorStartRaster	X X X.
CursorEndRaster	X X X.
StartAddressHigh	X X X.
StartAddressLow	X X X.
CursorHigh	X X X.
CursorLow	X X X.
LightPenHigh	X X X.
LightPenLow	X X X.