Difference between revisions of "C't 512 KB internal RAM expansion"

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Upgrade of the CPC 6128's internal RAM to 512 KiB as proposed in the article '''Aus David wird Goliath: 512 KB RAM für Schneider CPC''' in issue 10/1987, pages 156 to 162 of the German IT magazine '''c't'''. The DIY project name is '''CPC 6512'''. The mod requires to remove (or disable) the sixteen RAM chips on the CPC6128 mainboard, and to replace them by bigger chips, plus whatever additional bank selection logic.
+
[[Category:DIY]] [[Category:Memory expansions]]
 +
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''.
  
−
One issue later, c't 11/1987, the magazine published a driver which creates a RAM disk within the additional memory. This RAM disk can be used in [[AMSDOS]].
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== The Circuit ==
  
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== Technical ==
+
* '''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).
  
−
The memory is accessed via Port 7Fxxh, as on [[Standard Memory Expansions]]. Since the internal RAM is removed, its total capacity is only 512K (unlike standard 512K expansions which provide 64K internal plus 512K external memory, ie. 576K total). Altogether, the c't expansion works like a 448K dk'tronics expansion (leaving the banks selected via OUT [7Fxxh],FCh..FFh unused).
+
* '''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).
  
−
One half (256K) of the total (512K) expansion memory can be accessed as Video RAM (with normal expansions, only 64 KB are accessible as VRAM). This is making the expansion slightly incompatible with the dk'tronics standard. However, usually expansion memory is mapped to 4000h-7FFFh, whilst VRAM is usually mapped to C000h-FFFFh. So, most existing software may work with it, without accidently displaying garbage on the screen.
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<gallery>
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File:CPC6512 ct magazin schematic.png|Schematic
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== Installation Guid ==
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File:CPC6512 ct magazin pcb.jpg|PCB
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</gallery>
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=== Components ===
+
  
 +
'''Component List'''
 
   16x 41256 (256Kx1 DRAM)  ;replacing the CPC6128's 16 built-in 64Kx1 DRAMs
 
   16x 41256 (256Kx1 DRAM)  ;replacing the CPC6128's 16 built-in 64Kx1 DRAMs
−
   1x PAL 16L8              ;replacing the CPC6128's 16 built-in PAL
+
   1x PAL 16L8              ;replacing the CPC6128's built-in PAL
−
   1x 74LS38 (Quad 2-input NAND, OC)
+
   1x 74LS38               ;Quad 2-input NAND, OC
−
   1x 74LS273 (8bit latch, of which only 6bit are used here)
+
   1x 74LS273               ;8bit latch, of which only 5bit are used here
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   1x 2200 Ohm resistor
+
   1x 2200 Ohm resistor     ;pull-up for OC output
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   1x 470 Ohm resistor
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   1x 470 Ohm resistor     ;pull-up for OC output
−
   1x 47 Ohm resistor
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   1x 47 Ohm resistor       ;purpose unknown (noise related? shortcut protection?)
  
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=== Step 1 - Replacing the RAM ===
+
== Compatibility Problems ==
  
−
The old 64Kx1 DRAMs have same pin-outs as the new 256Kx1 ones (except, pin 1 was NC on old chips, on new chips it's A8 of RAS:CAS addresses; aka A16,A17 of linear addresses).
+
* 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 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, resulting in 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.
  
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* Replace the old RAMs by the new RAMs.
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== Memory Configurations ==
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* Wire pin 1 (formerly NC) of all RAMs to GND (allowing to test the circuit after Step 1) (later on, it will be wired to RAS:CAS A8 in Step 2).
+
  
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=== Step 2 - Daughterboard Logic ===
+
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).
  
−
The original article has been confusing on this part. It basically said that one needs "four additional connections", but without giving too clear info on which signals are to be connected to which locations.
+
  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
  
−
* Replace the old PAL by the new daughterboard (containing the new PAL, and the 2 logic chips and 3 resistors).
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== PAL Source Code ==
  
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== Missing Info ==
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Below is a typed-up and commented copy of the PALASM source code from the original article.
  
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* Unknown which banks are usable as VRAM
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D7D6 D0  D3  D4  D1  D2  NCAS A15  A14  GND  ;pin 1..10
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** The first four 16K bank are (probably) usuable as VRAM (as on all 64K CPC models)
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CPU  A15S AMUX MUX  LCLK CAS1 CAS0 IOWR A14S VCC  ;pin 11..20
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** The next four 16K bank are (hopefully) not usuable as VRAM (for CP/M+ compatibility, which uses them as Work RAM)
+
−
** For further banks it's totally unknown if they are used as VRAM or not
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IF (VCC) /LCLK= D7D6 * /A15 * /IOWR              ;load external latch on OUT [7Fxxh],C0h..FFh
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* Unknown if it's fully dk'tronics compatible, or more like incompletely implemented Inicron variant
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* Unknown what happens on accessing the unused region via OUT [7Fxxh],FCh..FFh
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IF (VCC) /CAS0= /NCAS * /D4  +                    ;bank bit4=0, select bank 0..15 (CPU and CRTC)
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* Chipset / Schematic is unknown
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          /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)
  
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== Newer Info ==
+
'''Note:''' For VRAM access, bank bits 2,3,4 are forced to zero (by above formulas), bank bit 0,1 are coming from the CRTC, passed directly to IC109 (without going through the PAL), so, the RAM banking affects only the CPU's memory accesses, not the CRTC's video memory accesses.
  
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Having a short look at the article, it seems to be all different as than expected:
+
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).
−
* There is no dk'tronics compatibility intended - however, c't and dk'tronics are based on the CPC6128 banking mechanism, so they work similar, and to some level there is some (unintended) compatibility. Namely, ONE HALF of the memory can be accessed in dk'tronics style fashion.
+
* 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 seems to be no hardware based "Video RAM" support at all. However, there's some unspectacular software based example included that copies expansion RAM to VRAM via LDIR opcode.
+
* 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 ==
 
== Scanned Article / Schematics ==
  
−
* [[Media:CPC6512 ct magazin part 1.pdf]] - c't issue 10/1987 - Schematic
+
* [[Media:CPC6512 ct magazin part 1.pdf]] - c't issue 10/1987 - RAM Expansion Schematic
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* [[Media:CPC6512 ct magazin part 2.pdf]] - c't issue 11/1987 - RAM Disc driver
+
* [[Media:CPC6512 ct magazin part 2.pdf]] - c't issue 11/1987 - RAM Disc driver for CP/M Plus
  
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== Offtopic ==
+
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.
  
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The '''CPC 6512''' c't ''Memory Expansion'' is not to be confused with issues '''CPC 6512''' and '''CPC 6513''' of the more popular ''World War II Fanzine'' from Concord Publications Company.
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== Related hardware modifications ==
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<gallery>
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File:Cpc6512.jpg|CPC 6512
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File:Cpc6513.jpg|CPC 6513
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</gallery>
+
  
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== Related hardware modifications ==
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* [[CPC_6320_/_CPC_6512_-_internal_320K_/_512K_for_CPC_6128|CPC 6320 / CPC 6512 (by eto) - similar logic, but fully standards compatible]]
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* [http://cpcwiki.eu/forum/index.php/topic,662.msg7113.html#msg7113 Bank Swapper by Khany/Cherry-T] (less complex: bank-exchanging toggle, no memory expansion; 1993)
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* [https://cpcwiki.eu/forum/index.php/topic,662.msg7113.html#msg7113 Bank Swapper by Khany/Cherry-T] (less complex: bank-exchanging toggle, no memory expansion; 1993)
 
* [[CPC4MB| CPC4MB memory upgrade by Yarek]] (more advanced; 2005/2006)
 
* [[CPC4MB| CPC4MB memory upgrade by Yarek]] (more advanced; 2005/2006)

Latest revision as of 09:10, 19 July 2023

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 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, resulting in 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)

Note: For VRAM access, bank bits 2,3,4 are forced to zero (by above formulas), bank bit 0,1 are coming from the CRTC, passed directly to IC109 (without going through the PAL), so, the RAM banking affects only the CPU's memory accesses, not the CRTC's video memory accesses.

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

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

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