{"id":1315,"date":"2019-05-19T11:24:29","date_gmt":"2019-05-19T09:24:29","guid":{"rendered":"https:\/\/www.pagetable.com\/?p=1315"},"modified":"2019-05-19T11:24:29","modified_gmt":"2019-05-19T09:24:29","slug":"run-cp-m-on-your-c64-using-emulation","status":"publish","type":"post","link":"https:\/\/www.pagetable.com\/?p=1315","title":{"rendered":"Run CP\/M on your C64 – using Emulation!"},"content":{"rendered":"

In this episode of computer archeology, we deconstruct a very interesting case of borrowing code from multiple places – but first try this D64 disk image with any Commodore 64 emulator or a real C64:<\/p>\n

\"\"
\nCommodore 64 CP\/M Emulator<\/a><\/p>\n

The CP\/M operating system was the first home computer OS that ran on machines from many different vendors, as long as they had an Intel 8080 compatible CPU – which the 8 bit machines from Commodore, Apple, Atari and BBC didn’t: They used the MOS 6502 CPU.<\/p>\n

The Commodore 64 CPM Cartridge<\/h3>\n

In 1983, Commodore released the the “CP\/M Cartridge<\/a>” for the Commodore 64, which contained a Z80 CPU and came with CP\/M 2.2: The OS and its apps would run on the Z80, which would switch over to the computer’s 6502 to call code in the original ROM for screen, keyboard and disk accesses. It wasn’t very good, didn’t sell well, and was quickly discontinued.<\/p>\n

Ro\u00dfm\u00f6ller’s CP\/M Emulator<\/h3>\n

But it was also possible to run CP\/M on a C64 without a hardware extension: All that is necessary is to emulate the Intel 8080 instruction set on the MOS 6502. And the German company Ro\u00dfm\u00f6ller did exactly that. In 1988, a little while after releasing their 4 MHz speeder cartridge “Turbo Process<\/a>” for the C64, they started selling<\/p>\n

\n
\n

CP\/M Emulator – CP\/M 2.2-kompatibler C64 ohne zus\u00e4tzl. Hardware<\/p>\n<\/blockquote>\n<\/blockquote>\n

for 10 DM.<\/p>\n

The product is directly based on Commodore’s adaption of CP\/M for the C64 CP\/M cartridge. Commodore’s disk contains a small bootloader that loads BIOS and the Z80 bootstrap from the first sectors on disk (as specified by CP\/M) – the bootloader of the CP\/M Emulator directly contains slightly patched versions of BIOS and the Z80 bootstrap, as well as the 8080 emulator:<\/p>\n\n\n\n\n
CP\/M Cartridge<\/th>\nCP\/M Emulator<\/th>\n<\/tr>\n
\n
0 \"CP\/M DISK       \" 65 2A\n1     \"CPM             \"  PRG\n0 BLOCKS FREE.\n<\/pre>\n<\/td>\n
\n
0 \"CP\/M EMULATOR   \" 32 0B\n12    \"CP\/M            \"  PRG\n0 BLOCKS FREE.\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
The first sectors on disk still contain the original versions of BIOS and the Z80 bootstrap – and curiously, all references on disk to “Digital Research”, the CP\/M license holder, have been replaced with “Ro\u00dfm\u00f6ller”, hinting towards a very questionable licensing status – not inconsistent with Ro\u00dfm\u00f6ller’s reputation at the time.<\/p>\n\n\n\n\n
CP\/M Cartridge<\/th>\nCP\/M Emulator<\/th>\n<\/tr>\n
\n
\n    COMMODORE 64   44k CP\/M vers 2.2\n\n  Copyright (C) 1979, Digital Research\n     Copyright (C) 1982, Commodore\n\nA>\n<\/pre>\n<\/td>\n
\n
     R 03 07\n\n    COMMODORE 64  44k CP\/M vers 2.2\n\n    ===============================\n\n    Copyright (C) 1988, ROSSMOELLER\n\nA>\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Emulator Performance<\/h3>\n
The Ro\u00dfm\u00f6ller 8080 emulator contained in the boot program is less than 1.4 KB in size – and it’s super slow: Executing a single 8080 instruction takes between 300 and 800 cycles (485 on average). This makes the CP\/M emulator about 100 times slower than a similarly spec’ed 8080-based machine. Even with the 8 MHz speeder, the emulator was still 12 times slower. It was in fact so slow that it failed to register normal keypresses on stock C64 systems unless you held the keys for a second.<\/p>\n
8080 Simulator for the 6502<\/h3>\n
There is a simple reason the emulator is this slow: It wasn’t written for speed. It wasn’t even written as an emulator. As it turns out, the Ro\u00dfm\u00f6ller 8080 emulator is a binary-patched version of the 8080 Simulator for the 6502, KIM-1 Version<\/em><\/a> “8080 simulator-debug package<\/em>” by Dann McCreary, written in 1978.<\/p>\n
8080 Simulator for the 6502<\/em> is an interactive 8080 simulator primarily aimed at teaching 8080 assembly – and optimized for size, since it had to to fit into the 2 KB of RAM of the KIM-1.<\/p>\n
In the following implementation of the memory access instructions STAX\/LDAX, you can see that the Ro\u00dfm\u00f6ller version just relocated everything by $CA pages:<\/p>\n\n\n\n\n
8080 Simulator for the 6502<\/th>\nRo\u00dfm\u00f6ller CP\/M Emulator<\/th>\n<\/tr>\n
\n
0249 DIRECT  PHA         ; TEMP SAVE\n024A         JSR PC\/PNT  ; PC TO POINTER\n024D         JSR DBLINC\n0250         LDXIM (SCR)\n0252         JSR MEM\/RP\n0255         LDXIM (PNT)\n0257         JSR SRC\/RP\n025A         PLA         ; RECOVER TEMP\n025B         LSRA        ; LOAD?\n025C         BCS STORE   ; NO, STORE\n025E         ASLA        ; MAKE INDEX\n025F         TAX\n0260         BEQ LDAD    ; LOAD A DIRECT?\n0262         JMP MEM\/RP  ; NO, LOAD HL DIRECT\n<\/pre>\n<\/td>\n
\n
.,CC49 48       PHA\n.,CC4A 20 5D CD JSR $CD5D\n.,CC4D 20 80 CD JSR $CD80\n.,CC50 A2 19    LDX #$19\n.,CC52 20 A0 CD JSR $CDA0\n.,CC55 A2 17    LDX #$17\n.,CC57 20 70 CD JSR $CD70\n.,CC5A 68       PLA\n.,CC5B 4A       LSR A\n.,CC5C B0 0E    BCS $CC6C\n.,CC5E 0A       ASL A\n.,CC5F AA       TAX\n.,CC60 F0 03    BEQ $CC65\n.,CC62 4C A0 CD JMP $CDA0\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Ro\u00dfm\u00f6ller Changes<\/h3>\n
But there are also sections in the emulator that had to be adapted for use in the CP\/M context:<\/p>\n