From ef7b3f76f0c57e78a237e6dc5690974eac1d4a66 Mon Sep 17 00:00:00 2001
From: John Winans <john@winans.org>
Date: Thu, 4 May 2023 21:48:53 -0500
Subject: [PATCH] If valid, use current drive after WBOOT else A

---
 retro/disk_callgate.asm | 13 ++++++++-----
 retro/retro.asm         | 13 ++++++++-----
 2 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/retro/disk_callgate.asm b/retro/disk_callgate.asm
index 8f7bd3d..dd089a1 100644
--- a/retro/disk_callgate.asm
+++ b/retro/disk_callgate.asm
@@ -77,18 +77,20 @@ endif
 ;****************************************************************************
 ;****************************************************************************
 disk_seldsk:
-        ld      a,c
-        ld      (disk_disk),a
 
 if .disk_debug >= 2
+	push	bc
 	call    iputs
 	db	"disk_seldsk entered\r\n\0"
-	ld	a,(disk_disk)		; restore the drive number
+	pop	bc
 endif
 	; check if the disk is valid
+        ld      a,c
 	cp	dph_vec_num
 	jr	nc,.seldsk_fail		; if (a >= dph_vec_num) then error
 
+        ld      (disk_disk),a
+
 	; disk is valid, find the DPH
 	ld	hl,dph_vec		; default = invalid
 	sla	a			; A = A * 2
@@ -183,7 +185,7 @@ if 0
 	call	hexdump	
 endif
 
-	pop	hl
+	pop	hl		; HL = DPH
 	ret
 
 
@@ -193,7 +195,8 @@ endif
 	ld	(disk_dph),hl
 	ld	(.cur_disk_read),hl
 	ld	(.cur_disk_write),hl
-	ret
+
+	ret			; HL = 0 = fail
 
 
 ;****************************************************************************
diff --git a/retro/retro.asm b/retro/retro.asm
index 4f75678..fdb59e2 100644
--- a/retro/retro.asm
+++ b/retro/retro.asm
@@ -322,11 +322,14 @@ if .debug >= 3
 	call	hexdump
 endif
 
-if 0
-	; This is not quite right because it include the user number and
-	; can get us stuck re-selesting an invalid disk drive!
-	ld	a,(4)		; load the current disk # from page-zero into a/c
-	and	0x0f		; the drive number is in the 4 lsbs
+if 1
+	ld	c,0		; default = drive A (if previous was invalid)
+	ld	a,(4)           ; load the current disk # from page-zero into A
+	and	0x0f            ; the drive number is in the 4 lsbs
+	cp	dph_vec_num
+	jp	nc,CPM_BASE     ; if A >= dph_vec_num then bad drive (use 0)
+
+	ld	a,(4)           ; load the current disk # from page-zero into a/c
 	ld	c,a
 else
 	ld	c,0		; The ONLY valid drive WE have is A!