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Subject[PATCH] setup.S: fix for BIOS/DOS not reporting ext mem size
Linus, Alan,

This patch to i386 setup.S resulted from my attempts to load big
bzImages from DOS. Loadlin fails at aroung 1020000 bytes, I didn't
want to mess with that much asm in loadlin src and wrote new
loader from scratch mostly in c.

I had a problem booting from DOS on pc with 196 mb or ram. It turned
out int15/e801 and int15/88 reported zero ext mem -> "Less than 4MB"
type hang. Digging in loadlin sources turned out some horrors
(loadlin hooks itself on int 15/88 and reads CMOS)

I think it's cleaner to do this in setup.S rather than jump thru the
hoops in DOS loader. Attached patch does that *only if* int15 methods
fail. It does not break existing practice, only adds just another
fallback in case of brain damage.

Tested on three boxes. Works for me. Applies cleanly to 2.4.13.
If something is wrong with this patch, let me know about it.

diff -ubB setup.S.orig setup.S
--- setup.S.orig Mon Sep 24 00:58:58 2001
+++ setup.S Wed Oct 31 12:21:45 2001
@@ -39,9 +39,13 @@
* from Ralf Brown interrupt list seem to indicate AX/BX should be used
* anyway. So to avoid breaking many machines (presumably there was a reason
* to orginally use CX/DX instead of AX/BX), we do a kludge to see
- * if CX/DX have been changed in the e801 call and if so use AX/BX .
+ * if CX/DX have been changed in the e801 call and if so use AX/BX.
* Michael Miller, April 2001 <>
+ * Even more fixes for memory detection when started from DOS
+ * TODO: maybe we can just resort to memory scan on our own
+ * to stop using int15/e820,int15/e801,int15/88,cmos once and for all?
+ * <> october 2001

#include <linux/config.h>
@@ -297,13 +301,13 @@

#define SMAP 0x534d4150

xorl %ebx, %ebx # continuation counter
movw $E820MAP, %di # point into the whitelist
# so we can have the bios
# directly write into it.

movl $0x0000e820, %eax # e820, upper word zeroed
movl $SMAP, %edx # ascii 'SMAP'
movl $20, %ecx # size of the e820rec
@@ -327,12 +331,10 @@
jnl bail820

incb (E820NR)
- movw %di, %ax
- addw $20, %ax
- movw %ax, %di
+ addw $20, %di
- cmpl $0, %ebx # check to see if
- jne jmpe820 # %ebx is set to EOF
+ testl %ebx, %ebx # check to see if
+ jnz jmp820 # %ebx is set to EOF

@@ -344,10 +346,10 @@
# alternative new memory detection scheme, and it's sensible
# to write everything into the same place.)

stc # fix to work around buggy
- xorw %cx,%cx # BIOSes which dont clear/set
- xorw %dx,%dx # carry on pass/error of
+ xorw %cx, %cx # BIOSes which dont clear/set
+ xorw %dx, %dx # carry on pass/error of
# e801h memory size call
# or merely pass cx,dx though
# without changing them.
@@ -355,28 +357,62 @@
int $0x15
jc mem88

- cmpw $0x0, %cx # Kludge to handle BIOSes
- jne e801usecxdx # which report their extended
- cmpw $0x0, %dx # memory in AX/BX rather than
- jne e801usecxdx # CX/DX. The spec I have read
+ testw %cx, %cx # Kludge to handle BIOSes
+ jnz e801use_cxdx # which report their extended
+ testw %dx, %dx # memory in AX/BX rather than
+ jnz e801use_cxdx # CX/DX. The spec I have read
movw %ax, %cx # seems to indicate AX/BX
movw %bx, %dx # are more reasonable anyway...

- andl $0xffff, %edx # clear sign extend
+ movzwl %dx, %edx # clear sign extend
shll $6, %edx # and go from 64k to 1k chunks
- movl %edx, (0x1e0) # store extended memory size
- andl $0xffff, %ecx # clear sign extend
- addl %ecx, (0x1e0) # and add lower memory into
- # total size.
+ movzwl %cx, %ecx # clear sign extend
+ addl %ecx, %edx # add lower memory to
+ movl %edx, (0x1e0) # extended, store

# Ye Olde Traditional Methode. Returns the memory size (up to 16mb or
# 64mb, depending on the bios) in ax.

+ #stc # guard against brain damage -
+ # int 15 must clear cf to indicate success
+ clc # unbelievable: some BIOSes/DOSes can leave
+ # cf as is so I had to abandon stc trick
movb $0x88, %ah
int $0x15
+ jc mem_cmos
+ testw %ax, %ax
+ jnz mem_store
+# Fallback: if int15/88 fails, get same data from CMOS
+# this works around extremely stupid case I had with PC not booting
+# from DOS when I put 196 megs of RAM in it
+# (it reported 0 via int15/e801 and int15/88)
+# Also this makes unnecessary for loadlin to jump thru the hoops
+# just in order to let us know ext mem size
+# (it hooks itself on int 15/88 and does CMOS reads for us)
+ pushf
+ cli
+ movb $0x18, %al
+ outb %al, $0x70
+ #iodelay?
+ inb $0x71, %al
+ movb %al, %ah
+ #iodelay?
+ movb $0x17, %al
+ outb %al, $0x70
+ #iodelay?
+ inb $0x71, %al
+ popf
+ cmpw $0xffff-0x400, %ax # We want to be sure it won't roll over
+ jbe mem_store2 # 16 bit value when low 1024k gets added
+ movw $0xffff-0x400, %ax # thus max memtop is 0xffff*1k = 64m-1k
movw %ax, (2)

# Set the keyboard repeat rate to the max
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