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| 6502 asm - what’s with all the jumping around? |
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| nali:
Quite simply back then memory address decoding was done with discrete logic from the 16 address lines A0 through A15. If the system was designed only to ever need half the 64K then why bother decoding all the address lines? So the memory isn't deliberately "mapped" as such, it's just an artefact of not using the most significant hardware address line. Write down the two addresses in binary, then erase the leftmost digit. |
| David Hess:
--- Quote from: eti on August 17, 2021, 05:52:14 am --- --- Quote from: retiredfeline on August 17, 2021, 05:40:50 am ---Nothing inherent in the CPU. Chances are that system has the lower 32kB mapped at the upper 32kB also. --- End quote --- Ah! Now, what does that mean, please? I’m a real newcomer. Thanks ever so. :)) --- End quote --- It means that when the installed memory is only a fraction of the address space, there is little reason to fully decode the address so the more significant bits of the address get ignored rather than decoded. This leads to the memory being duplicated at multiple addresses. |
| Benta:
Deleted. |
| RJSV:
Basic level, and middle-complex 6502: Your brief questions I will answer, just PM your 6502 question, mostly on instruction actions. The previous answers help with specific machines, using 6502 IC plus using the 'Signal' type. I think, historically linked to 6800 family type (loosely ?). But sure, ask any 6502, I am mid-level exp. (Wrote 'XEVIOUS' C-64, but that was 35 years ago !). Might take few days for responses, thanks. Rick |
| gbaddeley:
The Apple ][ had a 16 bit address space, with the monitor / AppleSoft basic rom at the top end, so that it had the reset/ int vectors in rom. Below the rom was addr space for video and I/O. Below that was ram for programs. (this is a simplified view). I still have my Apple ][ from 1983. |
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