| General > General Technical Chat |
| 6502 asm - what’s with all the jumping around? |
| (1/2) > >> |
| eti:
Hello. I’m new to 6502 asm and machine code, forgive my ignorance. Reset vector looks in fffc-ffffd for two bytes which are the start execution address. Now, elsewhere online, I see that this is also related to 7ffc-7ffd, but can’t fathom quite why. Also, why is address 8000 linked to address 0000? I’m sure it’s to do with memory mapping, but would be delighted if someone could clear it up in a clear and simple manner for me. Many Thanks indeed. |
| retiredfeline:
Nothing inherent in the CPU. Chances are that system has the lower 32kB mapped at the upper 32kB also. |
| eti:
--- 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. :)) |
| retiredfeline:
That usually happens when the memory space only partly used so they just omit using the upper address lines, A15 in this case, in decoding so the memory appears in two ranges of addresses. Sometimes bank switching is involved. It all depends on the particular system. |
| MarkF:
My first computer was an Apple ][ with a 6502 processor. Some where in upper memory (in the area you're asking about) was a hole (i.e. no physical memory) in the memory. That area included space to address the video memory, the floppy drive and address space for the I/O cards. I would have to go dig out the old manuals to find the memory allocation (which I still have ;D ). And the Apple still works! I don't remember how or where the interrupt vectors work any more. You need to find a 6502 datasheet. Oh, and there is also the boot ROM and BASIC interpreter in upper memory space. |
| Navigation |
| Message Index |
| Next page |