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A Blast From the Past: Emulating the Burroughs 220

Learn how to emulate the Burroughs B220 on an Apple II, including addressing, memory, instructions, I/O, assembly with Merlin, and more.

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A Blast From the Past: Emulating the Burroughs 220

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  1. A Blast From the Past:Emulating the Burroughs 220 Michael Mahon http://michaeljmahon.com

  2. Or…Plan 9 From Outer Space:The Resurrection of the Dead (Ed Wood)

  3. The Burroughs 220

  4. The Burroughs 220

  5. The Burroughs 220

  6. Burroughs 220 Users

  7. Another Burroughs 220 User...

  8. Inside the Burroughs 220

  9. The Burroughs 220 Console

  10. The Simulated Burroughs 220

  11. Simulating the B220on an Apple II • Apple II (6502) • 8-bit binary bytes • Binary addressing • 48KB memory • ASCII • Disk I/O • Burroughs 220 • 44-bit BCD words • BCD addressing • 5000 word memory • B220 code • Paper tape • Mag tape

  12. B220 Memory S 1 2 3 4 5 6 7 8 9 0 • 5000 words, Sign digit + 10 BCD digits • Each word is simulated as 6 BCD bytes • To map a B220 address to an Apple II address: • Convert 4-digit BCD to 16-bit binary • Check for non-BCD digits • Multiply by 6 • Add MEM base address That’s a LOT!

  13. B220 Addressing • BUT, it can all be done with tables! • Apple II address of B220 word is the 16-bit sum of two addresses in four tables indexed by the “raw” 2-byte BCD B220 address • Requires 36 cycles, including error checking! This made it practical to proceed.

  14. B220 Addressing AD00 tables 00DR tables A D D R } } 00 00 + . . . . . . . . . . . . Address $49 $99 Just two indexed loads and adds!

  15. B220 Architecture A R S 1 2 3 4 5 6 7 8 9 0 S 1 2 3 4 5 6 7 8 9 0 S V V V V O P A D D R B C P B220 Instructions: CAD, LDR, LDB; STA, STR, STB, STP IBB, DBB, IFL, DFL, DLB, BRP BFA, BFR; CFA, BCE, BCU, BCL, BCH; BUN ADD, SUB, MUL, DIV FAD, FSU, FMU, FDV PRD, PRB, PWR; MTR, MTW 7 8 9 0 7 8 9 0

  16. B220 I/O in ProDOS • BASIC.SYSTEM’s BSAVE and BLOAD are extremely general. • Using the “B” parameter, a single file can be used as a byte-addressable virtual memory. • Since ProDOS files can be sparse, a single file can simulate a large number of pseudo-files, even from Applesoft. • B220 paper tape units are represented as ProDOS binary files: “PTUNITn”.

  17. Archaeology

  18. B220 Assembly in Merlin • Macros dramatically increase the versatility of assemblers. • Enough to permit a 6502 assembler to assemble code for a word-oriented BCD machine! • Even though Merlin’s macro capability is relatively weak: • No optional parameters • Peculiar variable scoping • No ability to manipulate strings, etc., etc.

  19. B220 Assembly in Merlin 11 * Prime number sieve (03/30/64 - MJM) 12 13 start clb 14 test cad num ; Get next number. 15 srt 10 16 div -table ; Divide by prime. 17 bfr nxnum;00;0 ; Num isn't prime. 18 cfa -table ; Is quotient < divisor? 19 bcl addon ; -Yes, it's prime! 20 ibb test;1 ; -No, try next divisor. 21 addon cad tabln ; (Changed by quit) 22 cfaf hitab;04 23 bce quit ; Table full. 24 ifl tabln;00;1 ; Add prime to table. 25 ldb tabln 26 cad num 27 staw -table 28 xwrit cad num 29 call write 30 nxnum ifl num;00;2 ; Next odd number 31 bof stop 32 bun start ; Go again.

  20. B220 Assembly in Merlin 50 addr mac ; Label 51 if -,]1 ; B-modification 52 ]adn equ ]1 ; -Yes, make positive. 53 ]ad equ 0-]adn-z/6 54 else 55 ]ad equ ]1-z/6 56 fin 57 err ]ad/5000 ; Address limit 58 ]a4 equ ]ad/1000 ; Thousands digit 59 ]a4m equ ]a4*1000 60 ]a4r equ ]ad-]a4m 61 ]a3 equ ]a4r/100 ; Hundreds digit 62 ]a3m equ ]a3*100 63 ]a3r equ ]a4r-]a3m 64 ]a2 equ ]a3r/10 ; Tens digit 65 ]a2m equ ]a2*10 66 ]a1 equ ]a3r-]a2m ; Units digit 67 exp on 68 db ]a4*16+]a3,]a2*16+]a1 69 exp off 70 eom

  21. B220 Assembly in Merlin 11 * Prime number sieve (03/30/64 - MJM) 12 13 start clb 2000: 00 00 04 13 db ]sign,0,$4,$45 2003: 45 2004: 00 00 13 db ]a4*16+]a3,]a2*16+]a1 14 test cad num ; Get next number. 2006: 00 00 00 14 db ]sign,0,$0,$10 2009: 10 200A: 00 29 14 db ]a4*16+]a3,]a2*16+]a1 15 srt 10 200C: 00 00 01 15 db ]signs,0,$1,$48,0,$10 200F: 48 00 10 16 div -table ; Divide by prime. 2012: 01 00 00 16 db ]sign,0,$0,$15 2015: 15 2016: 01 42 16 db ]a4*16+]a3,]a2*16+]a1 17 bfr nxnum;00;0 ; Num isn't prime. 2018: 00 00 00 17 db ]sign,$00,$0,$37 201B: 37 201C: 00 18 17 db ]a4*16+]a3,]a2*16+]a1

  22. DEMO!

  23. Questions and discussion...

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