Questions tagged [6800]

Motorola 6800 family of processors. This does not include the similarly-numbered but different 6809 family.

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Behavior of lesser known illegal M6800 opcodes

The M6800 had a few illegal opcodes, including the (in)famous HCF (Halt and Catch Fire). The following illegal opcodes were revealed in BYTE Magazine, December 1977: $14: NBA $87: STAA immediate $C7: ...
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1answer
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What are the added opcodes for MC6801/MC6803?

What new opcodes were added to Motorola MC6801/MC6803? Background for the question, and what I've figured out so far (correct me if I'm wrong): The Motorola MC6801 (and MC6803) had an "enhanced ...
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474 views

What 6800 assembler to use?

Does anyone know of a good, modern Motorola 6800 assembler? I've used dasm, which I quite like (and it's even actively developed), but it doesn't actually support 6800. It supports 6803, which is ...
11
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1answer
280 views

Inverting the carry flag on an M6800

Are there any tricks that can be used to invert the carry flag on the 6800, in as few bytes as possible? This is the shortest subroutine I've come up with, which is straight forward: INVC: BCS ...
6
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1answer
299 views

How did the 6809 achieve source-code compatibility with the 6800?

The 6809 has 59 instructions, as compared with 78 on the 6800. So far as I can see, the 6809 lacks instructions that the 6800 has, such as aba, eorb and the transfer instructions (tba, tap and the ...
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2answers
157 views

How did the 6800's comparison operations work?

Does anyone have a quick primer to the way the 6800's compare-and-branch instructions worked internally? I'm referring to the instructions like BLS that compared a value in memory to the accumulator. ...
13
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6answers
2k views

Was the MC6800 a PDP-11 or PDP-8?

According to Gordon Bell, the 6800 was based on the PDP-11. According to Chuck Peddle, it was a PDP-8. Can anyone with knowledge of the PDP's pass judgement?
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How do I examine and use the SWTBUG monitor ROM in the SIMH SWTP6800 simulator?

I'd like to examine (disassemble and memory dump) and run the SWTBUG monitor ROM in the SIMH SWTP6800 emulator, but I seem to be having difficulty getting the devices attached properly, or I'm lost in ...
16
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1answer
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What's the timeline of Motorola 6800-family CPUs from 1974 to 1979?

According to Wikipedia, The Motorola 6800 was released in 1974. Between that and the release of the 6809 in 1979 (or 1978?) there was at least one other 6800-series CPU released, the 6802 ...
3
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1answer
311 views

What is a more featureful and/or better coded 6800 disassembler for modern machines than f9dasm?

I'm currently using [this version of f9dasm] on Linux to disassemble a moderately large Motorola 6800 program (Altair 680 Basic, about 8 KB in size). f9dasm isn't too bad, but I'd prefer to use ...
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289 views

Are there any CPUs or MCUs object-compatible with the 6800 that have additional index registers or addressing modes?

Having only a single index register and no addressing mode indexing against 16-bit addresses in memory seems to have been widely considered a primary reason that the Motorola 6800 fared poorly in ...
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Were people building CPUs out of TTL logic prior to the 4004, 8080 and the 6800?

I've just finished reading Charles Petzold's book, Code: The Hidden Language of Computer Hardware and Software. In it Charles explains building relays into gates, gates into logic components, and ...