multiboot: Don't forget last mmap entry

When the BIOS returns ebx = 0, the current entry is still valid and
needs to be included in the Multiboot memory map.

Fixing this meant that using bx as the entry index doesn't work any
more because it's 0 on the last entry (and it was SeaBIOS-specific
anyway), so the whole loop had to change a bit and should be more
generic as a result (ebx can be an arbitrary continuation number now,
and the entry size returned by the BIOS is used instead of hard-coding
20 bytes).
Signed-off-by: NKevin Wolf <mail@kevin-wolf.de>
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Message-id: 1372018066-21822-2-git-send-email-mail@kevin-wolf.de
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

pc-bios/optionrom/multiboot.S

@@ -89,17 +89,14 @@ run_multiboot:
 
 	/* Initialize multiboot mmap structs using int 0x15(e820) */
 	xor		%ebx, %ebx
-	/* mmap start after first size */
-	movl		$4, %edi
+	/* Start storing mmap data at %es:0 */
+	xor		%edi, %edi
 
 mmap_loop:
+	/* The multiboot entry size has offset -4, so leave some space */
+	add		$4, %di
 	/* entry size (mmap struct) & max buffer size (int15) */
 	movl		$20, %ecx
-	/* store entry size */
-	/* old as(1) doesn't like this insn so emit the bytes instead:
-	movl		%ecx, %es:-4(%edi)
-	*/
-	.dc.b		0x26,0x67,0x66,0x89,0x4f,0xfc
 	/* e820 */
 	movl		$0x0000e820, %eax
 	/* 'SMAP' magic */
@@ -107,21 +104,23 @@ mmap_loop:
 	int		$0x15
 
 mmap_check_entry:
-	/* last entry? then we're done */
+	/* Error or last entry already done? */
 	jb		mmap_done
-	and		%bx, %bx
-	jz		mmap_done
-	/* valid entry, so let's loop on */
 
 mmap_store_entry:
-	/* %ax = entry_number * 24 */
-	mov		$24, %ax
-	mul		%bx
-	mov		%ax, %di
+	/* store entry size */
+	/* old as(1) doesn't like this insn so emit the bytes instead:
+	movl		%ecx, %es:-4(%edi)
+	*/
+	.dc.b		0x26,0x67,0x66,0x89,0x4f,0xfc
+
+	/* %edi += entry_size, store as mbs_mmap_length */
+	add		%ecx, %edi
 	movw		%di, %fs:0x2c
-	/* %di = 4 + (entry_number * 24) */
-	add		$4, %di
-	jmp		mmap_loop
+
+	/* Continuation value 0 means last entry */
+	test		%ebx, %ebx
+	jnz		mmap_loop
 
 mmap_done:
 real_to_prot: