A project to upgrade a CPC6128 to 512K RAM (total), done by replacing the CPC6128's RAM chips, and its PAL banking chip, the project was released 1987 in german magazine c't Magazin.
The Circuit
- DRAM Replacement - Replace the sixteen old 64Kx1 DRAM chips (IC119-IC134) by new 256Kx1 DRAMs. The chips have same pinouts, only pin 1 is changed (old: NC, new: A8 row:column address signal). Optionally, replace only bank 0 (IC127-IC134) for getting only 320K (256K+64K) instead of 512K total (2x256K).
- PAL Replacement - Replace the old PAL (IC118) by the new daughterboard. Most signals connect to the PAL socket (green in schematic), a extra few wires connect elsewhere on mainboard (red), as an optional "bonus" feature, the circuit contains logic for a 3rd floppy drive (blue).
Schematic
PCB
Component List
16x 41256 (256Kx1 DRAM) ;replacing the CPC6128's 16 built-in 64Kx1 DRAMs 1x PAL 16L8 ;replacing the CPC6128's built-in PAL 1x 74LS38 ;Quad 2-input NAND, OC 1x 74LS273 ;8bit latch, of which only 5bit are used here 1x 2200 Ohm resistor ;pull-up for OC output 1x 470 Ohm resistor ;pull-up for OC output 1x 47 Ohm resistor ;purpose unknown (noise related? shortcut protection?)
Compatibility Problems
- RAMDIS is not supported, not a problem in most cases, but won't work with some special types of expansion hardware, like Multiface II.
- The bank selection for CPU address 4000h..7FFFh does also affect bank bits0-1 for VRAM at 4000h..7FFFh; thus the CRTC may see bank 0-3 in that region (whilst, as far as known, on a normal CPC, it should always see bank 1 in that region).
- Notice the included sample program does seem to map VRAM to 4000h, whilst mapping expansion banks to 4000h at the same time. Theoretically, this should result in display glitches - unknown if/how/why the sample works [?]
- The expansion seems to have been designed independently of Standard Memory Expansions like the dk'tronics one. So, there is no (intended) compatibility. However, both c't and dk'tronics are designed around the CPC6128 banking mechanism, so they do work similar in some ways. Namely, both can map expansion banks to 4000h..7FFFh, but there are several differences:
- For the mapping to 4000h..7FFFh, dk'tronics uses values CCh..CFh, D4h..C7h, DCh..DFh, ... FCh..FFh whilst c't uses values C8h..DFh. Some of these values do overlap, providing some (unintended) semi-compatibility, but there's still one big difference: along with the above values, dk'tronics maps bank 3 to C000h..FFFFh, whilst c't maps bank 7 to that region.
Memory Configurations
The memory is controlled by OUT [7Fxxh],C0h..DFh instructions. Values C0h..C7h are working same as on normal CPC6128s. Values C8h..DFh do access the additional RAM banks (C8h..D3h when using the cut-down 320K upgrade variant).
OUT [7Fxxh],C0h+... 0 1 2 3 4 5 6 7 8..31 ------------------------------------------------------- Bank at C000h..FFFFh 3 7 7 7 3 3 3 3 7 (!) Bank at 8000h..BFFFh 2 2 6 2 2 2 2 2 2 Bank at 4000h..7FFFh 1 1 5 3 4 5 6 7 8..31 Bank at 0000h..3FFFh 0 0 4 0 0 0 0 0 0
PAL Source Code
Below is a typed-up and commented copy of the PALASM source code from the original article.
D7D6 D0 D3 D4 D1 D2 NCAS A15 A14 GND ;pin 1..10
CPU A15S AMUX MUX LCLK CAS1 CAS0 IOWR A14S VCC ;pin 11..20
IF (VCC) /LCLK= D7D6 * /A15 * /IOWR ;load external latch on OUT [7Fxxh],C0h..FFh
IF (VCC) /CAS0= /NCAS * /D4 + ;bank bit4=0, select bank 0..15 (CPU and CRTC)
/CAS0= /NCAS * A15 +
/CAS0= /NCAS * /A14 +
/CAS0= /NCAS * CPU
IF (VCC) /CAS1= /NCAS * D4 * /A15 * A14 * /CPU ;bank bit4=1, select bank 16..31 (CPU at 4000h..7FFFh only)
IF (VCC) /A14S= /A14 + ;bank bit0
/D0 * D2 * /A15 +
/D0 * D3 * /A15 +
/D0 * D4 * /A15
IF (VCC) /A15S= /A14 * /A15 + ;bank bit1
/D1 * /A15 +
/D4 * /D3 * /D2 * /D0 * /A15 +
/D4 * /D3 * /D2 * /D1 * /A15
IF (VCC) /AMUX= /D0 * D1 * /D2 * /D3 * /D4 * /CPU * /MUX +
A15 * A14 * /D2 * D0 * /CPU * /MUX +
A15 * A14 * /D2 * D1 * /CPU * /MUX +
A15 * A14 * D3 * /CPU * /MUX +
A15 * A14 * D4 * /CPU * /MUX +
/A15 * A14 * D2 * /CPU * /MUX + ;bank bit2
/A15 * A14 * D3 * /CPU * MUX ;bank bit3
IF (GND) /MUX = /MUX ;dummy (do not output anything on this pin)
IF (GND) /IOWR=/IOWR ;dummy (do not output anything on this pin)
Some notes on the syntax:
- The first two lines assign the pin-outs. Observe that leading "/" slashes are omitted here. For example, "/CPU" (in schematic) becomes "CPU" (in source code). Accordingly "/CPU" (in source code) would be double-negated "//CPU" aka "CPU" (in schematic).
- The "IF (condition) signal=" part means that "signal" becomes an output when condition is true. In the above source code, condition is always true (VCC), or, for the last 2 lines, always false (GND).
- The "*" and "+" operators are meaning "* = AND", "+ = OR". The idea behind that confusing syntax was to make it "easier" to learn for people who are trained only in basic maths (the formulas do also work when treating *=multiply, and +=plus. For example: 1+0+1+1 = 3 = nonzero = true).
- There must be some priority ordering in the formulas: Either * before +, or operations inside of a line before merging the results of the separate lines (in above examples, both ordering methods do work).
Scanned Article / Schematics
- Media:CPC6512 ct magazin part 1.pdf - c't issue 10/1987 - RAM Expansion Schematic
- Media:CPC6512 ct magazin part 2.pdf - c't issue 11/1987 - RAM Disc driver for CP/M Plus
Original names of the articles are: Aus David wird Goliath: 512 KB RAM für Schneider CPC (part 1), and Byte-Hirte: RAM-Disk-Treiber für des CPCs 512 Kbyte (part 2). Both articles are written by Gabor Herr and Hubert Schröer.
Related hardware modifications
- Bank Swapper by Khany/Cherry-T (less complex: bank-exchanging toggle, no memory expansion; 1993)
- CPC4MB memory upgrade by Yarek (more advanced; 2005/2006)