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Explain how the JSR call is handled in some impls

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Peter Evans 2018-02-18 19:49:34 -06:00
parent 0fccf552f2
commit 00854e74b9
1 changed files with 9 additions and 0 deletions
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data/disk2.asm
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@ -112,6 +112,15 @@
46:AA TAX 46:AA TAX
47:BD 81 C0 LDA PHASEON,X 47:BD 81 C0 LDA PHASEON,X
4A:A9 56 LDA #$56 4A:A9 56 LDA #$56
; In at least one implementation (notably WinApple), the opcode below is
; rewritten as `A9 00 EA`, which is equivalent to:
; LDA #$00
; NOP
; This would essentially remove the WAIT call. The WAIT subroutine will,
; in the course of its operation, leave $00 in A, which explains the LDA
; #$00 opcode sequence. The NOP is there to replace the third byte
; (which was part of the JSR address in its original form).
4C:20 A8 FC JSR WAIT ; wait for the motor 4C:20 A8 FC JSR WAIT ; wait for the motor
4F:88 DEY 4F:88 DEY
50:10 EB BPL PHASELOOP 50:10 EB BPL PHASELOOP