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C't 512 KB internal RAM expansion

2,241 bytes added, 01:23, 17 December 2010
Upgrade of [[Category:DIY]] [[Category:Memory expansions]]A project to upgrade a CPC6128 to 512K RAM (total), done by replacing the CPC 6128CPC6128's internal RAM to 512 KiB as proposed in chips, and its PAL banking chip, the article '''Aus David wird Goliath: 512 KB RAM für Schneider CPC''' project was released 1987 in issue 10/1987, pages 156 to 162 of the German IT german magazine '''c'tMagazin'''. 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.
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]]. == The Circuit ==
== 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]]* '''PAL Replacement''' - Replace the old PAL (IC118) by the new daughterboard. Since Most signals connect to the internal RAM is removed, its total capacity is only 512K PAL socket (unlike standard 512K expansions which provide 64K internal plus 512K external memorygreen in schematic), ie. 576K totala extra few wires connect elsewhere on mainboard (red). Altogether, as an optional "bonus" feature, the c't expansion works like circuit contains logic for a 448K dk'tronics expansion 3rd floppy drive (leaving the banks selected via OUT [7Fxxh],FCh..FFh unusedblue).
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)<gallery>File:CPC6512 ct magazin schematic. This is making the expansion slightly incompatible with the dk'tronics standardpng|SchematicFile:CPC6512 ct magazin pcb. 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. == Installation Guide ==jpg|PCB=== Components ===</gallery>
'''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 47 Ohm resistor ;purpose unknown (noise related? shortcut protection?)
* '''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).== 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'PAL Replacementt and dk''' - Replace tronics are designed around the old PAL by the new daughterboardCPC6128 banking mechanism, so they do work similar in some ways. Most signals connect Namely, both can map expansion banks to 4000h..7FFFh, but there are several differences:** For the PAL socket (green in schematic)mapping to 4000h..7FFFh, a extra few wires connect elsewhere on mainboard (red)dk'tronics uses values CCh..CFh, as an optional "bonus" featureD4h..C7h, the circuit contains logic for a 3rd floppy drive DCh..DFh, ... FCh..FFh whilst c't uses values C8h..DFh. Some of these values do overlap, resulting in some (blueunintended)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.
<gallery>File:CPC6512 ct magazin schematic.png|SchematicFile:CPC6512 ct magazin pcb.jpg|PCB</gallery>== Memory Configurations ==
== Compatibility Problems ==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).
* RAMDIS is not supported, not a problem in most cases, but won't work with some special types of expansion hardware, like OUT [[Multiface II]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
== Missing Info PAL Source Code ==
* Unknown which banks are usable as VRAM** The first four 16K bank are (probably) usuable as VRAM (as on all 64K CPC models)** 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* Unknown if it's fully dk'tronics compatible, or more like incompletely implemented Inicron variant* Unknown what happens on accessing Below is a typed-up and commented copy of the unused region via OUT [7Fxxh],FChPALASM source code from the original article..FFh* Chipset / Schematic is unknown
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= Newer Info /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)
Having a short look at '''Note:''' For VRAM access, bank bits 2,3,4 are forced to zero (by above formulas), bank bit 0,1 are coming from the articleCRTC, it seems passed directly to be all different as than expected: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:* There is no dk'tronics compatibility intended The first two lines assign the pin- however, c't and dk'tronics outs. Observe that leading "/" slashes are based on the CPC6128 banking mechanismomitted here. For example, so they work similar, and to some level there is some "/CPU" (unintendedin schematic) becomes "CPU" (in source code) compatibility. Namely, ONE HALF of the memory can Accordingly "/CPU" (in source code) would be accessed double-negated "//CPU" aka "CPU" (in dk'tronics style fashionschematic). * There seems to be no hardware based The "Video RAMIF (condition) signal=" support at allpart means that "signal" becomes an output when condition is true. HoweverIn the above source code, there's some unspectacular software based example included 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 copies expansion RAM confusing syntax was to VRAM via LDIR opcodemake 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 driverfor 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 ==
 
* [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)
* [[CPC4MB| CPC4MB memory upgrade by Yarek]] (more advanced; 2005/2006)
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