PLM1800 and CDOS – A Blast From The Past

Thanks to a ton of work by a bunch of people on the Cosmac Elf mailing list, we have resurrected an operating system and a couple of compilers from the early days of the 1802.  Just getting the files off the old 3.5″ diskettes was a feat in itself. I think David Schultz did the work for Microdos and BASIC and Hank Riley for PLM. David went on to work through the file format and adapt an emulator to support it – His work is here. Marcel Tongren integrated the diskette support into the Emma 02 Emulator.

The PLM compiler compiles a PL/1-like language and was meant to run on the RCA Microdos system with 60K of memory.  I’m running microdos in the Emma 02 emulator and it’s a treat with a full set of dos-type commands: DIR, DEL, COPY etc besides the BASIC and PLM compilers and the ASM8 assembler.

DO;
  DECLARE T BYTE INITIAL(1);
  DO WHILE T>0;
    Q=1;
    CALL TIME(32767);
    Q=0;
    CALL TIME(32767);
  END;
END;
EOF

Compiling the program is a multi-step process. You run the compiler, merge the compiler output with the PLMLIB support routines, assemble the result with ASM8, then use a utility program CDSBIN to extract the binary code from the listing. The :1’s and :2’s are telling MicroDOS which drive the files are on :0 is the operating system, :1 is the PLM disk, and :2 is emulated on a windows directory.  The last couple of commands copy  the output .BIN and .LST files to the windows directory.

On windows I converted the raw binary in the .BIN file to intel hex and the uploaded it to the olduino with avrdude.  The blink program uses a PLM built-in routine TIME which is generates a delay of 90+10N where N is the parameter.  By happy coincidence 90+327670 cycles is around 1/2 sec on my 4mhz olduino so it makes a nice blink as you can see below. I think everyone working on the recovery of MicroDOS and the compilers has been working on emulators so I imagine this is the first time that PLM code has run on hardware since the 80’s.

The code, at first glance looks pretty basic. Seems to be mostly generated subroutine calls rather than inline code, maybe a stack machine underneath?

0409 ;             0160
0409 ;             0161 MAIN   ORG  $
0409 ;             0162 ..00001 01 DO;
0409 ;             0163 ..00002 02
  DECLARE T BYTE INITIAL(1);
0409 ;             0164 Z09000  EQU       $
0409 ;             0165        ORG       Z10000
046A 01;           0166          DC       00001
046B ;             0167        ORG       Z09000
0409 ;             0168 ..00003 02
  DO WHILE T>0;
0409 ;             0169 Z01000 ORG  $
0409 D40287;       0170        CALL LOAD1
040C 046A;         0171          DC      Z10000
040E D40255;       0172        CALL COMPOP
0411 044F;         0173          DC      Z00050
0413 CB041E;       0174        LBNF     $+00011
0416 D40063;       0175        CALL SMOP
0419 044F;         0176          DC      Z00050
041B D4026F;       0177        CALL TEST
041E C0044C;       0178        LBR       Z01001
0421 ;             0179 Z01002 ORG  $
0421 ;             0180 ..00004 03
    Q=1;
0421 D4022D;       0181        CALL LOADOP
0424 0451;         0182          DC      Z00051
0426 8F;           0183        GLO  RF
0427 F6;           0184        SHR
0428 CB042F;       0185        LBNF     $+00007
042B 7B;           0186        SEQ
042C C00430;       0187        LBR      $+00004
042F 7A;           0188        REQ
0430 ;             0189 ..00005 03
    CALL TIME(32767);
0430 D40236;       0190        CALL TIMDEL
0433 0455;         0191          DC      Z00053
0435 ;             0192 ..00006 03
    Q=0;
0435 D4022D;       0193        CALL LOADOP
0438 044F;         0194          DC      Z00050
043A 8F;           0195        GLO  RF
043B F6;           0196        SHR
043C CB0443;       0197        LBNF     $+00007
043F 7B;           0198        SEQ
0440 C00444;       0199        LBR      $+00004
0443 7A;           0200        REQ
0444 ;             0201 ..00007 03
    CALL TIME(32767);
0444 D40236;       0202        CALL TIMDEL
0447 0455;         0203          DC      Z00053
0449 ;             0204 ..00008 03
  END;
0449 C00409;       0205        LBR       Z01000
044C ;             0206 Z09001  EQU       $
044C ;             0207        ORG       Z00053
0455 7FFF;         0208          DC       32767
0457 ;             0209        ORG       Z09001
044C ;             0210 Z01001 ORG  $
044C ;             0211 ..00009 02
END;
044C ;             0212 ..00010 01
EOF
044C C00409;       0213 Z00001 LBR        MAIN
044F 00;           0214 Z00050   DC 00H
0450 00;           0215          DC 0
0451 00;           0216 Z00051   DC 00H
0452 01;           0217          DC       00001
0453 00;           0218 Z00052   DC 00H
0454 02;           0219          DC       00002
0455 ;             0220 Z00053 ORG      $+00002
0457 ;             0221 Z00054 ORG      $+00002
0459 0000;         0222 Z00003   DC 0000H
045B 0000;         0223 Z00004   DC 0000H
045D 0000;         0224 Z00005   DC 0000H
045F 0000;         0225 Z00006   DC 0000H
0461 0000;         0226 Z00007   DC 0000H
0463 0000;         0227 Z00008   DC 0000H
0465 0000;         0228 Z00009   DC 0000H
0467 0000;         0229 Z00010   DC 0000H
0469 ;             0230 Z09999 ORG      $+00001
046A ;             0231 Z10000 ORG      $+00001
046B ;             0232        PAGE
0500 ;             0233        ORG      $+00255
05FF ;             0234 STACK  ORG      $+00001
0600 ;             0235 Z30000 ORG  $
0600 ;             0236 MEMORY ORG  $
0600 ;             0237        END 

 

Edited to change references to CDOS to MicroDOS. CDOS was an even earlier RCA operating system for the 1802 which did in fact use 8″ diskettes.