It is currently Thu Feb 20, 2020 4:05 pm

All times are UTC

Post new topic Reply to topic  [ 2 posts ] 
Author Message
 Post subject: PDP-8 emulator
PostPosted: Fri Jul 27, 2007 1:52 pm 

Joined: Fri Jul 20, 2007 12:33 pm
Posts: 8
Location: England
I've been working at 68k assembly on and off for about a week now, and have come up with something that runs PDP-8 code fairly well. Loads of things are still missing, and I haven't tested that every opcode I have added works, but ...
For those that don't know (maybe most of you :) ), the PDP-8 was one of the first minicomputers, from 1965 I think, made by DEC with a 12-bit wordlength. They say it's the perfect choice for anyone who wants to write an emulator and who don't have much experience in programming :) so maybe one day this project will grow into a fully-fledged emulator...

So here's the source, enjoy.

START   ORG   $1000
*************************************************************** INTERRUPTS ****
INTER   bclr   #$0,d7      deal with interrupts.
   beq   INT2
INT1   addq.w   #$2,a0      Interrupt mode 1. Skip next instruction.
INT2   bclr   #$1,d7      Interrupt mode 2. Execute a JMS 0000 instruction.
   beq   INT3
   move.w   #$0800,d0
   jmp   OPPARSE      
INT3   bclr   #$3,d7      Interrupt mode 3. Execute the command in r11.
   beq   INT4
   move   #$0800,d0
   jmp   OPPARSE
INT4   jmp   OPFETCH      Nothing happens. Just continue with the program.
************************************************************* FETCH OPCODE ****
OPFETCH   move.w   (a0)+,d0      get opcode. Inc PC
************************************************************ PARSE ADDRESS ****
ADPARSE   move.w   d0,d2
   lsl.w   #$1,d2      adress by byte, not word.
   andi.w   #$01FF,d2      work out effective adress.
   bclr   #$8,d2      check if zero page.
   beq   DIRECTION      that's what we have now, so check direction.
   move.w   a0,d4      otherwise, get page from left 5 bits of PC
   andi.w   #$0F800,d4
   or.w   d4,d2      that should do it
DIRECTION   move.w   d2,a1
   btst   #$8,d0      indirect?
   beq   OPPARSE
INDIRECT   move.w   d2,a2      now calculate effective adress
   move.w   (a2),d2
   lsl.w   #$1,d2      of course we need to adress by byte, not word.
   move.w   d2,a1
   cmpi.w   #$8,d2      autoinc reg?
   blt   OPPARSE
   cmpi.w   #$F,d2
   bgt   OPPARSE
   move   d2,a2      add 1 to this register.
   addq.w   #$1,(a2)
**************************************************** INVOKE OPCODE ROUTINE ****
OPPARSE   move.w   d0,d2      d2 will hold command for now
   lsr   #$8,d2
   lsr   #$1,d2
   andi.w   #$0007,d2
   cmpi   #$0007,d2      which command have we got here?
   beq   OPS1
   cmpi   #$0006,d2
   beq   IOTOP
   cmpi   #$0005,d2
   beq   JMPOP
   cmpi   #$0004,d2
   beq   JMSOP
   cmpi   #$0003,d2
   beq   DCAOP
   cmpi   #$0002,d2
   beq   ISZOP
   cmpi   #$0001,d2
   beq   TADOP
ANDOP   move.w   (a1),d2      AND operand to register
   and.w   d2,d1
   jmp   INTER
TADOP   add.w   (a1),d1      TAD - add operand to register
   jmp   INTER
ISZOP   move.w   (a1),d2      ISZ - increment operand by 1. Skip next instruction if it overflows to 0
   addq.w   #$1,d2
   andi.w   #$0fff,d2
   bne   NOSKIP
   bset   #$0,d7      set skip flag
NOSKIP   andi.w   #$f000,(a1)      rightmost twelve bits will always be 0 in this case
   or.w   d2,(a1)
   jmp   INTER
DCAOP   move.w   d1,(a1)      DCA - deposit and clear accumulator
   move.w   #$0,d1
   jmp   INTER
JMSOP   move.w   a0,(a1)+      JMS - store PC at word
   move.w   a1,a0      and continue execution from the following word
   addq   #$2,a0      execute next command after that.
   jmp   OPFETCH      in this case, don't allow an interrupt to be dealt with till next round
JMPOP   move.w   a1,a0      JMP - jump
   jmp   INTER
IOTOP   move   d0,d2      IOT - input output transfers.
   andi.w   #$8F,d2      put port and signal into d2 - 7 bits
   cmpi   #$0E,d2
   jmp   INTER
************************************************** MICROCODED OPERATIONS I ****
OPS1   btst   #$8,d0      OPR - micrcoded operations.
   bne   OPS2
OPS1CLA   btst   #$7,d0      CLA - clear AC
   bne   OPS1CLL
   andi.w   #$F000,d1
OPS1CLL   btst   #$6,d0      CLL - clear Link
   bne   OPS1CMA
   andi.w   #$0FFF,d1
OPS1CMA   btst   #$5,d0      CMA - one's complement AC
   bne   OPS1CML
   eori.w   #$0FFF,d1
OPS1CML   btst   #$4,d0      CML - one's complement Link
   bne   OPS1IAC
   eori.w   #$F000,d1
OPS1IAC   btst   #$0,d0      IAC - increment AC
   bne   OPS1BSW6
   addq.w   #$1,d1
OPS1BSW6   btst   #$2,d0      See what's to be done next.
   bne   OPS1RAL
   btst   #$3,d0
   bne   OPS1RAR
   btst   #$1,d0
   beq   EPILOGUE
   move.w   #$d1,d2      BSW6 - swap rightmost 6 bits of register.
   rol.w   #$2,d2      do this in d2
   ror.b   #$2,d2
   rol.w   #$8,d2
   rol.b   #$2,d2
   ror.w   #$2,d2
   andi.w   #$000f,d2      keep the link bits. these are still intact in d1
   andi.w   #$f000,d1
   or.w   d2,d1
   jmp   EPILOGUE
OPS1RAL   btst   #$3,d0
   bne   OPS1BSW8
   rol   #$1,d1      RAL - Rotate register left
   btst   #$1,d0
   beq   EPILOGUE
   rol   #$1,d1
   jmp   EPILOGUE
OPS1RAR   ror   #$1,d1      RAL - Rotate L,AC left
   btst   #$1,d0
   beq   EPILOGUE
   ror   #$1,d1
   jmp   EPILOGUE
OPS1BSW8   ror.w   #$8,d1      BSW - swap 8 bit bytes in AC.
   jmp   EPILOGUE
************************************************* MICROCODED OPERATIONS II ****
OPS2   btst   #$3,d0                           *OR SKIPS*
   bne   OPS2SKP
OPS2SMA   btst   #$6,d0      SMA - skip if reg<0
   beq   OPS2SZA
   btst   #$b,d1
   beq   OPS2SZA
   bset   #$0,d7
OPS2SZA   btst   #$5,d0      SZA - skip is reg=0
   beq   OPS2SNL
   move   #$d1,d4      will set flag Z accordingly
   beq   OPS2SNL
   bset   #$0,d7
OPS2SNL   btst   #$4,d0      SNL - skip if L<>0
   move   #$d1,d4
   andi.w   #$F000,d4
   beq   OPS2CLA
   bset   #$0,d7
   jmp   OPS2CLA
OPS2SKP   bset   #$0,d7                          *AND SKIPS*
   btst   #$6,d0      SKP - unconditional skip
   beq   OPS2SZL      SPA - skip if reg>0
   btst   #$b,d1
   bne   OPS2SZL
   bclr   #$0,d7
   bra   OPS2CLA
OPS2SZL   nop ***************      SZL - skip if L=0 - fill this in later
OPS2CLA   nop
***************************************************************** EPILOGUE ****


Fewer bicycles going rusty at the bottom of lakes,
More in everyday use. Go green!

 Post subject:
PostPosted: Wed Aug 06, 2008 1:45 pm 

Joined: Mon Dec 27, 2004 11:40 pm
Posts: 318
I made one too and put it here.

I can find diagnostic programs for the PDP-8 but they're all in paper tape format. I have no idea yet how co convert these into a raw binary image that could then be assembled into the simulated memory.

So until I do there's only a "hello world!" program.


Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 2 posts ] 

All times are UTC

Who is online

Users browsing this forum: No registered users and 1 guest

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group