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.
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.
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
See also
- Bit 14 of the PAL selection address must be at 1 on CPCs equipped with CRTCs 0, 1, 2. It can be at 0 or 1 on CRTCs 3 and 4. For compatibility reasons, it is strongly advised to always set bit 14 to 1 to select PAL.
- For RAM banking settings see Register 3 of the Gate Array (Note that no settings are stored in the gate array, but the PAL and gate array share an I/O port address).