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Retrocomputing

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    Brilliant, that answers the first part! Any idea about the "24 wait states"? (I’m inclined to mark as "accepted" and raise another question?) Commented Nov 18, 2024 at 9:07
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    @andrewf The NOPs might be connected to setting the I register, so you might want to wait some time before splitting your question. Commented Nov 18, 2024 at 9:41
  • I wonder why the ROM’s authors didn’t just do: XOR A; LD I,A …unless to make that "less than $40" more explicit. Commented Nov 18, 2024 at 10:41
  • @andrewf: In IM2, the interrupt routine jumps to the address contained at the word at address I*256+255 (or whatever is on the data bus at the time), and by setting it to $3F, it'll point to an area filled with $FF, leading it to jump to address $FFFF where you'd normally put a JR byte (can't remember opcode) which would then - combined with the opcode for the first byte in the ROM - a DI - would lead to a relative jump some bytes backward in memory, where you would then place an absolute JP to your interrupt routine. Perhaps the author thought of this when he decided to use $3F. Commented Nov 18, 2024 at 11:11
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    @HeartWare, That crossed my mind too, but the 257 0xFFs are at 0x3800, not 0x3F00, so it would need to put 0x38 into I not `0x3F. breakintoprogram.co.uk/hardware/computers/zx-spectrum/… (Not to mention the fact that the ROM uses IM 1, not IM 2.) Commented Nov 18, 2024 at 13:33