optimized MBR and added memmap identification

1 files changed, 55 insertions, 0 deletions

diff --git a/boot/memmap.asm b/boot/memmap.asm
new file mode 100644
index 0000000..29d48f3
--- /dev/null
+++ b/boot/memmap.asm
@@ -0,0 +1,55 @@
+;---------------------------------------------
+;	Get memory map from bios
+;	/in es:di->destination buffer for entries
+;	/ret bp=entry count
+;---------------------------------------------
+
+; stolen from: http://www.brokenthorn.com/Resources/OSDev17.html
+struc	MemoryMapEntry
+    .baseAddress	resq	1	; base address of address range
+    .length		resq	1	; length of address range in bytes
+    .type		resd	1	; type of address range
+    .acpi_null	resd	1	; reserved
+endstruc
+ 
+[bits 16]
+
+BiosGetMemoryMap:
+    pushad
+    xor	ebx, ebx
+    xor	bp, bp			; number of entries stored here
+    mov	edx, 'PAMS'		; 'SMAP'
+    mov	eax, 0xe820
+    mov	ecx, 24			; memory map entry struct is 24 bytes
+    int	0x15			; get first entry
+    jc	.error	
+    cmp	eax, 'PAMS'		; bios returns SMAP in eax
+    jne	.error
+    test	ebx, ebx		; if ebx=0 then list is one entry long; bail out
+    je	.error
+    jmp	.start
+.next_entry:
+    mov	edx, 'PAMS'		; some bios's trash this register
+    mov	ecx, 24			; entry is 24 bytes
+    mov	eax, 0xe820
+    int	0x15			; get next entry
+.start:
+    jcxz	.skip_entry		; if actual returned bytes is 0, skip entry
+.notext:
+    mov	ecx, [es:di + MemoryMapEntry.length]	; get length (low dword)
+    test	ecx, ecx		; if length is 0 skip it
+    jne	short .good_entry
+    mov	ecx, [es:di + MemoryMapEntry.length + 4]; get length (upper dword)
+    jecxz	.skip_entry		; if length is 0 skip it
+.good_entry:
+    inc	bp			; increment entry count
+    add	di, 24			; point di to next entry in buffer
+.skip_entry:
+    cmp	ebx, 0			; if ebx return is 0, list is done
+    jne	.next_entry		; get next entry
+    jmp	.done
+.error:
+    stc
+.done:
+    popad
+    ret