From 06864281d6bbf60614183a73f6efdab838221f09 Mon Sep 17 00:00:00 2001
From: DeaDDooMER <deaddoomer@deadsoftware.ru>
Date: Wed, 5 Aug 2020 23:57:22 +0300
Subject: [PATCH] boot0: add alternative method to get memory size for older
 machines

---
 Boot486/boot0.S | 79 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 58 insertions(+), 21 deletions(-)

diff --git a/Boot486/boot0.S b/Boot486/boot0.S
index ef70f6b..efc93c9 100644
--- a/Boot486/boot0.S
+++ b/Boot486/boot0.S
@@ -131,30 +131,67 @@ print:
 .code16
 mapmem:
 	pusha
+
 	movl $0, MemoryMapAdr
 	xor %ebx, %ebx
 	mov $(MemoryMapAdr + 4), %di
-	mapmem__loop:
-		mov $0xe820, %eax
-		mov $20, %ecx
-		mov $0x0534D4150, %edx
-		int $0x15
-		jc mapmem__failed
-		cmp %edx, %eax
-		jne mapmem__failed
-		cmp $0, %ebx
-		je mapmem__done
-		add $20, %di
-		incl MemoryMapAdr
-		jmp mapmem__loop
-	mapmem__done:
-		popa
-		ret # <============
-	mapmem__failed:
-		mov $msg_mapmem_failed, %si
-		call print
-		cli
-		hlt
+
+mapmem_loop:
+	mov $0xe820, %eax
+	mov $20, %ecx
+	mov $0x0534D4150, %edx
+	int $0x15
+	jc mapmem_method_88
+	cmp %edx, %eax
+	jne mapmem_method_88
+	cmp $0, %ebx
+	je mapmem_done
+	add $20, %di
+	incl MemoryMapAdr
+	jmp mapmem_loop
+
+mapmem_method_88:
+	#####
+	### function:
+	###   int 0x15 + %ah = 0x88
+	### input:
+	###   none
+	### output:
+	###   cf clear:
+	###     %ax = available memory starting at 0x100000 in KB
+	###   cf set:
+	###     %ah = error code
+	### note:
+	###   some bioses do not clear cf, do it manually
+        #####
+
+	movl $1,        (MemoryMapAdr + 0)  # num
+	movl $0x100000, (MemoryMapAdr + 4)  # base (low)
+	movl $0,        (MemoryMapAdr + 8)  # base (high)
+	movl $0,        (MemoryMapAdr + 12) # len (low)
+	movl $0,        (MemoryMapAdr + 16) # len (high)
+	movl $1,        (MemoryMapAdr + 20) # type
+
+        clc
+	mov $0x88, %ah
+	int $0x15
+	jc mapmem_failed
+	test %ax, %ax
+	je mapmem_failed
+	movzx %ax, %eax
+	shl $10, %eax
+	movl %eax, (MemoryMapAdr + 12) # len (low)
+	#jmp mapmem_done
+
+mapmem_done:
+	popa
+	ret # <============
+
+mapmem_failed:
+	mov $msg_mapmem_failed, %si
+	call print
+	cli
+	hlt
 
 # =========================================================================== #
 # Convert LBA to CHS
-- 
2.29.2