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Quick Question about ORG $F000 vs ORG $1000


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I didn't want to necro this post from 2002 ;-) , but in it, Dan says

Since the 6507 processor only 13 address lines, the highest memory location it can access is $1FFF. But this doesn't stop you from putting your code at $F000. If the 6507 encountered a JMP $F000 for example, the top 3 bits of the address would be dropped and the jump would go to $1000 instead.

So, if $F000 essentially points to $1000, why to VCS programmers always use ORG $F000 instead og ORG $1000? Is it more efficient or something? What's the difference?






(Link back to original post: http://atariage.com/forums/topic/3244-org-f000/?p=31613)

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I learned to code for the 6502 on the Commodore VIC 20, 64, and 128 back in the 80s. On the 6502 the vectors for NMI, RESET, and IRQ are by definition at $FFFA, $FFFC, and $FFFE respectively. So for me, setting it to $F000 (and $F800 for 2K games) makes the vectors the same as what I learned back in the day.

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No difference.


You can use $1000, $3000, $5000, $7000, $9000, $b000, $d000 or $f000.


I use $f000 just because it feels the right thing if the 6502 starts from vector at $fffc, even if the three higher bits of address never goes out thru real pins.


Besides we can never know if someday comes out an Atari VCS with the full 64K byte addressing and we need to be compatible.


Well, one can dare to dream :grin:.

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