From ae5cb754fb9540794bdb5465a77e1263ca257539 Mon Sep 17 00:00:00 2001
From: Oliver Schmidt <ol.sc@web.de>
Date: Mon, 8 Aug 2022 15:09:42 +0200
Subject: [PATCH] Avoid unwanted i/o access.

Depending on the 6502, LDA addr,X and/or STA addr,X produce additional phantom reads. In order to avoid those phantom reads to do i/o access, I used the "classic" approach of moving those reads from page $C0 to page $BF.
---
 Apple2/RPi.Command.asm | 18 ++++++++++++------
 Apple2/Shell.asm       | 17 +++++++++++------
 2 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/Apple2/RPi.Command.asm b/Apple2/RPi.Command.asm
index 56316e7..b5e1db9 100644
--- a/Apple2/RPi.Command.asm
+++ b/Apple2/RPi.Command.asm
@@ -17,10 +17,15 @@ XLEN       =  $BE52    ;length of command string-1.
 XCNUM      =  $BE53    ;CI cmd no. (ext cmd - 0).
 PBITS      =  $BE54    ;Command parameter bits.
 XRETURN    =  $FF58    ;Known RTS instruction.
-InputByte = $c08e
-OutputByte = $c08d
-InputFlags = $c08b
-OutputFlags = $c087
+
+; have 6502 absolute indexed access phantom reads 
+; happen on page $bf to avoid unwanted i/o access
+IndexOffset = $8f
+
+InputByte = $c08e - IndexOffset
+OutputByte = $c08d - IndexOffset
+InputFlags = $c08b - IndexOffset
+OutputFlags = $c087 - IndexOffset
 
 ReadBlockCommand = $01
 WriteBlockCommand = $02
@@ -76,9 +81,10 @@ DetectSlot:
  asl
  asl
  asl
- tax
  clc
- bcc Start
+ adc #IndexOffset
+ tax
+ bcc Start ;always
 nextSlot:
  dex
  bne DetectSlot
diff --git a/Apple2/Shell.asm b/Apple2/Shell.asm
index 7a613b3..67cd253 100755
--- a/Apple2/Shell.asm
+++ b/Apple2/Shell.asm
@@ -19,10 +19,14 @@ IOError = $27
 NoDevice = $28
 WriteProtect = $2B
 
-InputByte = $c08e
-OutputByte = $c08d
-InputFlags = $c08b
-OutputFlags = $c087
+; have 6502 absolute indexed access phantom reads 
+; happen on page $bf to avoid unwanted i/o access
+IndexOffset = $8f
+
+InputByte = $c08e - IndexOffset
+OutputByte = $c08d - IndexOffset
+InputFlags = $c08b - IndexOffset
+OutputFlags = $c087 - IndexOffset
 
 ResetCommand = $00
 ReadBlockCommand = $01
@@ -85,9 +89,10 @@ DetectSlot:
  asl
  asl
  asl
- tax
  clc
- bcc Init
+ adc #IndexOffset
+ tax
+ bcc Init ;always
 nextSlot:
  dex
  bne DetectSlot