Make debug hexprint work under PCem too

PCem emulating IBM PC 5150 seems to have a bug where int 0x13/2 will report
it read zero sectors regardless of how many sectors it actually read.

bootsect.asm

@@ -121,8 +121,15 @@ search_root:
 		je .end
 
 		test byte [si + 11], 0x08 + 0x10
-		jz .isfile
+		jnz .skipentry
 
+		; Make sure the file has non-zero size
+		cmp word [si + 28], 0
+		jne .isfile
+		cmp word [si + 30], 0
+		jne .isfile
+
+	.skipentry:
 		add si, 32
 		jmp .entry
 
@@ -154,18 +161,12 @@ search_root:
 	.end:
 
 notfound:
-	; TODO: Better error messages
+	mov si, notfound_msg
-	mov ax, 0x0e00 + '?'
+	jmp fatal_error
-	int 0x10
-
-hang:
-	hlt
-	jmp hang
 
 found:
 	; SI points to 11 bytes after the start of the entry, so adjust all
 	; offsets
-	; TODO: Handle zero-length files
 	mov ax, [si - 11 + 26] ; First cluster
 	push ax
 
@@ -248,8 +249,13 @@ loadsectors:
 	push ax
 	push cx
 	push dx
+	push di
 
 	.loop:
+		mov di, 3 ; Retry thrice
+
+		.retry:
+
 		push ax
 		push cx
 
@@ -282,45 +288,62 @@ loadsectors:
 			and al, 0xC0
 			or cl, al
 
-		; Copy number of sectors to still read to ax and cap at 127
+		mov ah, 2
-		pop ax
+		mov al, 1
-		push ax
+		mov dl, [drivenumber]
-		cmp al, 127
+		int 0x13
-		jbe .read
-		mov al, 127
 
-		.read:
+		; TODO: Handle reset & retry
-			mov ah, 2
+		jc .error
-			mov dl, [drivenumber]
-			int 0x13
 
-		; Decrement cx by the number of sectors read
 		pop cx
-		xor ah, ah
+		pop ax
-		sub cx, ax
 
-		; Increment bx by the number of sectors read times 512
+		inc ax
-		xor dx, dx
+		add bx, 512
-		mov dh, al
-		shl dx, 1
-		add bx, dx
 
-		; Increment ax by the number of sectors read
+		loop .loop
-		pop dx
-		add ax, dx
-
-		test cx, cx
-		jnz .loop
 
+	pop di
 	pop dx
 	pop cx
 	pop ax
 	ret
 
-%include "hexprint.inc"
+	.error:
+		; Do we still have retries remaining?
+		mov si, diskerror_msg
+		test di, di
+		jz fatal_error ; No, fail
+		dec di
+
+		; Yes, reset disk
+		xor ah, ah
+		int 0x13
+
+		; Execute the loop again
+		pop cx
+		pop ax
+		jmp .retry
+
+fatal_error:
+	lodsb
+	test al, al
+	jz hang
+	mov ah, 0xe
+	int 0x10
+	jmp fatal_error
+
+hang:
+	hlt
+	jmp hang
+
 
 kernel_name: db "KERNEL  BIN"
 
+notfound_msg: db "Kernel not found", 0
+diskerror_msg: db "Disk error", 0
+
 times 510-($-$$) db 0
 db 0x55, 0xaa
 _end:

hexprint.inc

@@ -10,18 +10,19 @@ hexprint8:
 	mov cl, al
 	xor bx, bx
 
-	mov ah, 0xe
 	shr al, 1
 	shr al, 1
 	shr al, 1
 	shr al, 1
 	mov bl, al
 	mov al, [.digits + bx]
+	mov ah, 0xe
 	int 0x10
 
 	mov bl, 0xf
 	and bl, cl
 	mov al, [.digits + bx]
+	mov ah, 0xe
 	int 0x10
 
 	pop cx