From 5a0272a52b3aa705f928a021c3b7a74a12fd2379 Mon Sep 17 00:00:00 2001
From: Ivo van Poorten <ivopvp@gmail.com>
Date: Thu, 16 Jan 2025 20:18:33 +0100
Subject: [PATCH] OSI: Fix RAM top detection. Real hardware does not read 0xff
 when there's no RAM, but it returns the MSB of the address. Test RAM with a
 value that is not equal to the MSB of the address.

---
 src/arch/osi/osi.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/arch/osi/osi.S b/src/arch/osi/osi.S
index 757a9b1f..fcb8157e 100644
--- a/src/arch/osi/osi.S
+++ b/src/arch/osi/osi.S
@@ -163,10 +163,12 @@ init:
         lda ptr+1
         cmp #$c0
         zbreakif_eq
+        lda #$3f
         sta (ptr),y
         cmp (ptr),y
     zuntil_ne
 
+    lda ptr+1
     sec
     sbc #PAGES_PER_TRACK
     sta ptrkbuf                     ; put track buffer at ramtop