Programmable Array Logic (PAL). Not to be confused with PAL, the colour encoding system for analog televisions.
The CPC6128 second bank of 64K RAM is controlled by a PAL 16L8 chip. It has the Amstrad part number 40031.
On the CPC 6128 schematic, it is top centre: CPC6128 Schematic however the X inputs aren't distinguished.
PAL MMR register
This register controls how the extended RAM is banked into the CPU address space. It doesn't affect the video display at all as the Gate Array can only access the Base 64k page of RAM.
| MMR | 64K page | S | MM | CPU Memory Mapping | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | &0000-&3fff | &4000-&7fff | &8000-&bfff | &c000-&ffff |
| 1 | 1 | x | 0 | 0 | 0 | Base 64k / Bank 0 | Base 64k / Bank 1 | Base 64k / Bank 2 | Base 64k / Bank 3 | ||
| 1 | 1 | p | 0 | 0 | 1 | Base 64k / Bank 0 | Base 64k / Bank 1 | Base 64k / Bank 2 | Page p / Bank 3 | ||
| 1 | 1 | p | 0 | 1 | 0 | Page p / Bank 0 | Page p / Bank 1 | Page p / Bank 2 | Page p / Bank 3 | ||
| 1 | 1 | p | 0 | 1 | 1 | Base 64k / Bank 0 | Base 64k / Bank 3 | Base 64k / Bank 2 | Page p / Bank 3 | ||
| 1 | 1 | p | 1 | b | Base 64k / Bank 0 | Page p / Bank b | Base 64k / Bank 2 | Base 64k / Bank 3 | |||
On a standard 128k machine (unexpanded), bits5..3 are ignored. Page will always be fixed to Page 0.
PAL I/O port
Note that no settings are stored in the Gate Array itself regarding the MMR register. But the PAL and Gate Array share an I/O port address so that it appears to be the same chip to the programmer.
Bit14 of the PAL selection address can be at 0 or 1 on CPCs equipped with CRTCs 3, 4. It must be at 1 on CRTCs 0, 1 and 2 (The result is not guaranteed).
For compatibility reasons, it is strongly advised to always set bit14 to 1 to select PAL.
Furthermore, if bit14=0 then CRTC will be selected too.
PAL Type Detection
10 OUT &7F00,&C0:POKE &4000,&C0 20 OUT &7F00,&C7:POKE &4000,&C7:OUT &7F00,&C0 30 IF PEEK(&4000)=&C7 THEN PRINT"PAL chip absent/inactive":END 40 OUT &3FFF,&C7 50 IF PEEK(&4000)=&C7 THEN PRINT"IO Bit14=0 PAL selected" ELSE PRINT"IO Bit14=0 PAL not selected" 60 OUT &7F00,&C0:POKE &C000,&C0 70 OUT &7F00,&C3:POKE &4000,&C3:OUT &7F00,&C0 80 IF PEEK(&C000)=&C3 THEN PRINT"Valid RAM mode &C3" ELSE PRINT"Invalid RAM mode &C3"
Note: The CPC 464/664 cannot deal with A14/A15 for Base 64k page like the 6128 does. So external RAM expansions differ in their behaviour regarding &C3 mode. See Discussion on the forum and Another discussion
Initial replacement equation (Porchy)
The following equations were worked out by Porchy (member on CPCWiki Forum). These can be used to program replacements:
A15OUT = (!X2 & !X1 & A14
# !X3 & !X2 & A14
# A15);
!X1 = (!A15 & D7ANDD6 & RESET & !IOWR & D0
# !X1 & RESET & IOWR
# !X1 & !D7ANDD6 & RESET
# !X1 & A15 & RESET);
!X2 = (!A15 & D7ANDD6 & RESET & !IOWR & D1
# !X2 & RESET & IOWR
# !X2 & !D7ANDD6 & RESET
# !X2 & A15 & RESET);
!X3 = (!A15 & D7ANDD6 & RESET & !IOWR & D2
# !X3 & RESET & IOWR
# !X3 & !D7ANDD6 & RESET
# !X3 & A15 & RESET);
!CAS1 = (X3 & !X1 & A15 & A14 & !NCAS & !RAMDIS & !CPU & CAS0
# !X3 & !A15 & A14 & !NCAS & !RAMDIS & !CPU & CAS0
# X3 & !X2 & X1 & !NCAS & !RAMDIS & !CPU & CAS0
# !NCAS & CAS0 & !CAS1);
!CAS0 = (X3 & X2 & X1 & !NCAS & !RAMDIS & CAS1
# X3 & !X1 & !A15 & !NCAS & !RAMDIS & CAS1
# !X3 & A15 & !NCAS & !RAMDIS & CAS1
# !X1 & !A14 & !NCAS & !RAMDIS & CAS1
# !X3 & !A14 & !NCAS & !RAMDIS & CAS1
# !NCAS & !RAMDIS & CPU & CAS1
# !NCAS & !CAS0 & CAS1);
A14OUT = (A15 & A14
# !X1 & A14
# X3 & A14);
File:Amstrad6128.jed Original JED File posted on CPCWiki Forum
Fixed version (Gerald)
Original version from Porchy suffer from a bad handling of the RAMDIS signal. This cause screen artefact when accessing an external extension RAM like XMEM.
A14OUT = !( !A14
# !A15 & !Q0 & Q2 );
A15OUT = !( !A15 & !A14
# !A15 & !Q1
# !A15 & !Q0 & !Q2 );
Q0 = ( D7ANDD6 & nRESET & D0 & !A15 & !nIOWR
# !D7ANDD6 & nRESET & Q0
# nRESET & A15 & Q0
# nRESET & nIOWR & Q0 );
Q1 = ( D7ANDD6 & nRESET & D1 & !A15 & !nIOWR
# !D7ANDD6 & nRESET & Q1
# nRESET & A15 & Q1
# nRESET & nIOWR & Q1 );
Q2 = ( D7ANDD6 & nRESET & D2 & !A15 & !nIOWR
# !D7ANDD6 & nRESET & Q2
# nRESET & A15 & Q2
# nRESET & nIOWR & Q2 );
nCAS0 = ( nCAS
# RAMDIS & !nCPU & nCAS0
# !A15 & A14 & !nCPU & Q2 & nCAS0
# A15 & A14 & !nCPU & Q0 & !Q2 & nCAS0
# !nCPU & !Q0 & Q1 & !Q2 & nCAS0
# !nCAS1 );
nCAS1 = !( !RAMDIS & !nCAS & !A15 & A14 & !nCPU & Q2 & nCAS0
# !RAMDIS & !nCAS & A15 & A14 & !nCPU & Q0 & !Q2 & nCAS0
# !RAMDIS & !nCAS & !nCPU & !Q0 & Q1 & !Q2 & nCAS0
# !nCAS & !A15 & A14 & Q2 & nCAS0 & !nCAS1
# !nCAS & A15 & A14 & Q0 & !Q2 & nCAS0 & !nCAS1
# !nCAS & !Q0 & Q1 & !Q2 & nCAS0 & !nCAS1 );
File:CPC6128.JED : Fixed version of Amstrad 40031 GAL replacement
File:CPC6128.hex : Fixed version of Amstrad 40031 GAL replacement, Hex Intel version.