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MOS 6502: A Blast From the Past

posted by Fnord666 on Thursday January 30 2020, @03:58AM   
from the not-the-lol-bug dept.

An Anonymous Coward writes:

https://www.pagetable.com/?p=406

The MOS 6502 CPU was introduced in September of 1975, and while the documentation described the three shift/rotate instructions ASL, LSR and ROL, the ROR instruction was missing – the documentation said that ROR would be available in chips starting in June 1976. In fact, the reason for this omission was that the instruction, while being present, didn't behave correctly. Only few 6502s with the defect are in existence, and nobody seemed to have checked what was actually going on in these chips.

Simon C got my KIM-1 working again, which has a 6502 from week 51 of 1975. There are 512 possible inputs to ROR (8 bit A plus 1 bit C; assuming it doesn't have dependencies on other registers), and roughly two bytes of output: the 8 bit result and the processor status (flags) register. We ran the following programs on the KIM-1 – note that we had to split the task into several programs, because the KIM-1 doesn't have enough RAM to hold all results.https://www.pagetable.com/?p=406

An Anonymous Coward writes:

http://www.cpushack.com/2020/01/14/barn-find-mos-mcs6502-a-restoration/

In car collecting one of the 'holy grail' experiences is the 'Barn Find' finding and recovering a rare vehicle that has sat untouched, in some barn, or shed for some time. They are often in rough, but original condition and can evoke much excitement. As it turns out CPUs are not so different. I recently purchased a very rough and very old ATARI Arcade board.

The pictures clearly showed it in terrible condition, with lots of oxidation and 'stuff' on it. But it also had a white MOS 6502 processor. These are some of the very first CPUs made by MOS and are rather desirable, as in addition to their use by ATARI, they were used in the very first Apple computer, the Apple 1.

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Original Submission 1Original Submission 2

• decompress decompress (Score: 2) by shortscreen on Thursday January 30 2020, @09:00AM (1 child)

  by shortscreen (2252) on Thursday January 30 2020, @09:00AM (#951099) Journal

  ; lameness filter is lame

          lda <$07        ; $07 and $08 contain destination address
          sta <$10
          lda <$08
          sta <$11
          ldy #0
          lda [$05],y     ; $05 and $06 contain source address
          sta <$12
          inc <$05
          bne a640
          inc <$06
          bne a640
  a657:
          inc <$06
          bne a639
  a640:
          lda [$05],y
          inc <$05
          beq a657
  a639:
          cmp <$12
          bne a641
          lda [$05],y
          beq alldone
          inc <$05
          bne a642
          inc <$06
  a642:
          cmp #1
          bne a644
          lda <$12
          sta [$07],y
          inc <$07
          bne a640
          inc <$08
          bne a640
  a644:
          tax
          lda [$05],y
          clc
          adc <$10
          sta <$B4
          inc <$05
          bne a645
          inc <$06
  a645:
          lda [$05],y
          adc <$11
          sta <$B5
          inc <$05
          bne a646
          inc <$06
  a646:
          lda [$B4],y
          sta [$07],y
          iny
          dex
          bne a646
          tya
          ldy #0
          clc
          adc <$07
          sta <$07
          bcc a640
          inc <$08
          bne a640
  a641:
          sta [$07],y
          inc <$07
          bne a640
          inc <$08
          jmp a640

  Starting Score:	1		point
  Karma-Bonus Modifier		+1
  Total Score:		2

• Re:decompress (Score: 2) by dry on Saturday February 01 2020, @06:52AM

  by dry (223) on Saturday February 01 2020, @06:52AM (#952253) Journal

  Weird assembly, the lesser sign represents zero page? Even the mini-assembler in the Apple II knew the difference between
  LDA $07 and LDA $0007.

  Parent