From 845ba3666c3b8ae91808c3bc39e09ec544945aac Mon Sep 17 00:00:00 2001
From: Matthieu Bucchianeri <mbucchia@gmail.com>
Date: Tue, 11 Feb 2025 19:05:50 -0800
Subject: [PATCH] Allow swapping driver ID between the BIOS disk and XTMax.

---
 XTMax/Code/XTMax/bootrom.h        |  4 +--
 XTMax/Drivers/BootROM/bootrom.asm | 51 +++++++++++++++++++++++++++++--
 XTMax/Drivers/BootROM/tests.inc   | 10 ++++--
 3 files changed, 58 insertions(+), 7 deletions(-)

diff --git a/XTMax/Code/XTMax/bootrom.h b/XTMax/Code/XTMax/bootrom.h
index 9a88888..237de61 100644
--- a/XTMax/Code/XTMax/bootrom.h
+++ b/XTMax/Code/XTMax/bootrom.h
@@ -19,7 +19,7 @@ unsigned char BOOTROM[] = {
     4, 184, 51, 231, 232, 56, 4, 251, 94, 90, 89, 88, 7, 203, 85, 86,
     128, 250, 128, 116, 57, 137, 197, 137, 214, 156, 14, 184, 47, 225, 80, 156,
     186, 132, 2, 237, 80, 186, 134, 2, 237, 80, 137, 232, 137, 242, 207, 156,
-    80, 137, 232, 128, 252, 8, 117, 18, 137, 240, 60, 128, 114, 12, 6, 184,
+    80, 137, 240, 60, 128, 114, 19, 137, 232, 128, 252, 8, 117, 12, 6, 184,
     64, 0, 142, 192, 38, 138, 22, 117, 0, 7, 88, 157, 235, 58, 128, 252,
     21, 126, 5, 232, 113, 0, 235, 34, 128, 252, 1, 116, 10, 128, 252, 21,
     116, 5, 190, 122, 225, 235, 3, 190, 136, 225, 86, 137, 222, 136, 227, 48,
@@ -127,4 +127,4 @@ unsigned char BOOTROM[] = {
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 177};
\ No newline at end of file
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 176};
\ No newline at end of file
diff --git a/XTMax/Drivers/BootROM/bootrom.asm b/XTMax/Drivers/BootROM/bootrom.asm
index 0734730..d3036a1 100644
--- a/XTMax/Drivers/BootROM/bootrom.asm
+++ b/XTMax/Drivers/BootROM/bootrom.asm
@@ -25,6 +25,14 @@ cpu 8086    ; ensure we remain compatible with 8086
 ;
 %define DISK_NUMBER         FIXED_DISK_0
 
+%if DISK_NUMBER == FIXED_DISK_0
+;
+; Whether we are going to rename the BIOS's 1st disk to be the second disk.
+; This is useful to allow booting from the second disk.
+;
+;%define TAKE_OVER_FIXED_DISK_0
+%endif
+
 ;
 ; Whether we will try/force using our own bootstrap code instead of falling back to BASIC.
 ;
@@ -178,6 +186,13 @@ entry:
     mov ax, newline
     call print_string
 
+;
+; Move fixed disk 0 to fixed disk 1.
+;
+%ifdef TAKE_OVER_FIXED_DISK_0
+    call swap_fixed_disk_parameters_tables
+%endif
+
 ;
 ; Install our fixed disk parameter table.
 ; For the 1st disk, it is stored in the interrupt vector table, at vector 41h.
@@ -283,6 +298,13 @@ int13h_entry:
 ; This is not an operation for the SD Card. Forward to the BIOS INT 13h handler.
 ;
 .forward_to_bios:
+%ifdef TAKE_OVER_FIXED_DISK_0
+    cmp dl, 0x80+FIXED_DISK_1   ; is this the other fixed drive?
+    jne .no_fixed_disk_take_over
+    mov dl, 0x80+FIXED_DISK_0
+    call swap_fixed_disk_parameters_tables
+.no_fixed_disk_take_over:
+%endif
     mov TEMP0, ax               ; save ax
     mov TEMP1, dx               ; save dx
     pushf                       ; setup for iret from INT 13h handler
@@ -314,12 +336,16 @@ int13h_entry:
 .return_from_int13h:
     pushf
     push ax
-    mov ax, TEMP0               ; original ax
-    cmp ah, 0x08                ; is read parameters?
-    jne .skip_update_hdnum
     mov ax, TEMP1               ; original dx
     cmp al, 0x80                ; is fixed fixed?
     jb .skip_update_hdnum
+%ifdef TAKE_OVER_FIXED_DISK_0
+    mov dl, 0x80+FIXED_DISK_1
+    call swap_fixed_disk_parameters_tables
+%endif
+    mov ax, TEMP0               ; original ax
+    cmp ah, 0x08                ; is read parameters?
+    jne .skip_update_hdnum
     push es
     mov ax, 0x40                ; BIOS data area
     mov es, ax
@@ -990,6 +1016,25 @@ int18h_entry:
     jmp .loop
 %endif
 
+%ifdef TAKE_OVER_FIXED_DISK_0
+swap_fixed_disk_parameters_tables:
+    push es
+    push ax
+    push bx
+    xor ax, ax
+    mov es, ax
+    mov ax, es:[(0x41+FIXED_DISK_0*5)*4+2]
+    xchg es:[(0x41+FIXED_DISK_1*5)*4+2], ax
+    mov es:[(0x41+FIXED_DISK_0*5)*4+2], ax
+    mov ax, es:[(0x41+FIXED_DISK_0*5)*4]
+    xchg es:[(0x41+FIXED_DISK_1*5)*4], ax
+    mov es:[(0x41+FIXED_DISK_0*5)*4], ax
+    pop bx
+    pop ax
+    pop es
+    ret
+%endif
+
 ;
 ; Compute LBA address based on CHS address
 ; in:  CH = cylinder number (low 8 bits)
diff --git a/XTMax/Drivers/BootROM/tests.inc b/XTMax/Drivers/BootROM/tests.inc
index cea7f5d..0b93828 100644
--- a/XTMax/Drivers/BootROM/tests.inc
+++ b/XTMax/Drivers/BootROM/tests.inc
@@ -16,7 +16,7 @@
     mov di, 0xbbcc
     mov bp, 0xddee
 
-    mov dl, 0x80
+    mov dl, 0x81
     mov ah, 0x08
 
     ;mov bx, buf_write
@@ -27,7 +27,7 @@
 
     call do_int13h
 
-    mov dl, 0x80
+    mov dl, 0x81
     mov ah, 0x02
 
     mov bx, ds
@@ -145,6 +145,11 @@ fake_int13h_entry:
     mov al, ah
     xor ah, ah
     call print_hex
+    mov ax, drive_msg
+    call print_string
+    mov ax, dx
+    xor ah, ah
+    call print_hex
     mov ax, newline
     call print_string
     pop ax
@@ -162,6 +167,7 @@ buf_write           db 1, 2, 3, 4, 5, 6, 7, 8
 buf_read            times 1024 db 0
 
 fake_handler_msg    db 'BIOS INT13h Function ', 0
+drive_msg           db ' Drive ', 0
 test_success_msg    db 'Call succeeded ', 0
 test_failed_msg     db 'Call failed ', 0