From 00854e74b9ca67c10067fdbc1d2e8e9813e1f779 Mon Sep 17 00:00:00 2001
From: Peter Evans <peter@modul8r.com>
Date: Sun, 18 Feb 2018 19:49:34 -0600
Subject: [PATCH] Explain how the JSR call is handled in some impls

---
 data/disk2.asm | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/data/disk2.asm b/data/disk2.asm
index 6dd4fd3..f4bfa3a 100644
--- a/data/disk2.asm
+++ b/data/disk2.asm
@@ -112,6 +112,15 @@
 46:AA                       TAX
 47:BD 81 C0                 LDA PHASEON,X
 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
 4F:88                       DEY
 50:10 EB                    BPL PHASELOOP