using int13h/8h only for floppies for now, geometry on emulated USB

disks seems to be completly wrong. Probing on real machine for
first non-readable sector hangs the machine or creates reboots, no
clue. Writing a boot loader is not the scope of the project. For
me it runs where it has to run for now..

3 files changed, 36 insertions, 19 deletions

diff --git a/src/Makefile b/src/Makefile
index fed9fd3..1609299 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -9,7 +9,7 @@ MAGIC := $(shell printf '%x' `date +%s`)
 all: image.bin kernel.sym
 
 # truncate to correct number of sectors, we have
-# 512 (boot, stage 1) + N * 512 (N currenty is 3, stage 2) = 2048 for boot.bin
+# 512 (boot, stage 1) + N * 512 (N currenty is 3, stage 2) = 3072 for boot.bin
 # + M * 512 (M is currently 7) = 3144 for kernel.bin
 # + 1 * 512 = 512 for magic.bin
 # (M + N + 1 is the number of sectors to be read in stage 2, as stage 1
@@ -102,6 +102,10 @@ run-qemu-hd: image.bin
 	qemu-system-i386 -net nic,model=ne2k_pci -d guest_errors -m 32 -drive "file=image.bin,if=ide,format=raw" \
 		-serial file:serial.log
 
+run-qemu-usb: image.bin
+	qemu-system-i386 -net nic,model=ne2k_pci -d guest_errors -m 32 -usb -usbdevice disk:/dev/sde \
+		-serial file:serial.log
+
 run-qemu: image.bin
 	qemu-system-i386 -net nic,model=ne2k_pci -d guest_errors -m 32 -drive "file=image.bin,if=floppy,format=raw" \
 		-serial file:serial.log
diff --git a/src/boot.asm b/src/boot.asm
index 73f1214..68cbb52 100644
--- a/src/boot.asm
+++ b/src/boot.asm
@@ -87,7 +87,11 @@ stage2:
 	mov si, MESSAGE_STAGE_2_LOADED
 	call print_string
 
-; detect disk geometry
+; detect disk geometry (works for real floppies only?)
+	mov cl, [BOOT_DRIVE]
+	and cl, 0x80
+	jnz .not_a_floppy
+
 	mov si, MESSAGE_DETECTING_DISK_GEOMETRY
 	call print_string
 	mov dl, [BOOT_DRIVE]
@@ -95,6 +99,7 @@ stage2:
 	mov dl, [BOOT_DRIVE]
 	call probe_and_fix_disk_geometry
 	call print_newline
+.not_a_floppy:
 
 ; print a message we are now loading the kernel
 	mov si, MESSAGE_LOADING_KERNEL
diff --git a/src/stage2_real_functions.asm b/src/stage2_real_functions.asm
index f173ca3..1728b1a 100644
--- a/src/stage2_real_functions.asm
+++ b/src/stage2_real_functions.asm
@@ -9,7 +9,7 @@ SECTORS_PER_CYLINDER:
 	db 0x3F				; detect parameters enters the correct value here (sectors + 1)
 					; if detection fails, force int13 to read ahead
 NOF_HEADS:
-	db 0x02				; number of heads + 1
+	db 0x01				; number of heads + 1
 SECTORS_TO_LOAD:
 	db NOF_LOAD_SECTORS		; load NOF_LOAD_SECTORS sectors in total
 CURRENT_SECTOR:
@@ -49,31 +49,39 @@ detect_disk_geometry:
 	ret
 
 probe_and_fix_disk_geometry:
-	mov cl, 0xff			; from 63 down to 0
+	mov cl, 0x1			; from 1 to 63
+	mov BYTE [SECTORS_PER_CYLINDER], cl
 
 .loop:
+	mov dl, [BOOT_DRIVE]
 	mov ah, 0x02			; read sectors from drive
 	mov al, 1			; read 1 sector
+	mov cl, BYTE [SECTORS_PER_CYLINDER]
 	mov ch, BYTE 0
 	mov dh, BYTE 0
 			
 	int 0x13
 	
-	jc .next_try
-					; no errors anymore, so it must be the correct sector
-	inc cl
-	mov [SECTORS_PER_CYLINDER], BYTE cl
-
-	mov al, '>'
-	call print_char
-	movzx dx, byte [SECTORS_PER_CYLINDER]
-	call print_hex
-
-	ret
-
-.next_try:
-	dec cl				; next sector
-	jnz .loop
+	jnc .next_sector		; on error, remeber cl being the highest sector
+
+.fix_heads:
+	cmp [NOF_HEADS], BYTE 0		; 0 heads, the BIOS is playing tricks to us!
+	je .illegal_heads
+	jmp .end
+
+.illegal_heads:
+	mov [NOF_HEADS], BYTE 1
+	jmp .end
+
+.next_sector:
+	mov cl, BYTE [SECTORS_PER_CYLINDER]
+	inc cl				; next sector
+	mov BYTE [SECTORS_PER_CYLINDER], cl
+	cmp cl, 0x40			; to a max of 63, if so, this is the default
+	jbe .loop
+	jmp .fix_heads
+
+.end:
 	ret
 
 ; read the whole stage2 and kernel from the disk