Skip to main content
Retrocomputing

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

9
  • The routine does an indirect jump to the contents of putchar_ptr, because rts jump to the contents just pushed on the stack. I don't know why they are doing it that way instead of using a plain indirect jump. Commented Oct 20, 2018 at 15:59
  • 1
    @dirkt Because the address is possibly stored in routine-1 form and ment to be used that way. An indirect Jump would require a non decremented one, So this is faster than loading and incrementing it for use with an indirect jump. Commented Oct 20, 2018 at 17:06
  • @Raffzahn: So why don't they just store it in addr instead of addr-1 form? It was certainly common to do that, e.g. COUT on the Apple II, and IIRC also in the C64 ROM. Commented Oct 20, 2018 at 17:27
  • @dirkt Because it's part of a list of addresses - or better list of IOCB. The Atari OS is a bit more sophisticated compared with the C64. It is based on an abstract, file based IO system called CIO, which used IO controll blocks for each open file. For BASIC there is a lisst of 8 IOCB predefined. Each has at offset 6 a pointer with the function to be called for output ($34*6* here). For outputing the routine is just called with the IOCB number times $10 in X, so instead of a fixed address, above LDA use (IOCB_list+6,X) to fetch whatever pointer is needed and uses the RTS mechanic. Commented Oct 20, 2018 at 17:34
  • 1
    @Raffzahn: Ah, so the real answer is that it's a not a single vector, but part of a data structure, and that's the reason the -1 form is used (by choice), because you can't use an indirect jump anyway, and you need to use pha/rts. Commented Oct 20, 2018 at 18:02