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

[vda]

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.

I couldn't resist and fixed some typos, cleaned up some label names
and replaced some instructions with shorter code :-)

Tested on three boxes. Works for me. :-)
-- 
Best regards, vda
mailto:[e-mail]

diff -ubB --expand-tabs setup.S.orig setup.S
--- setup.S.orig        Mon Sep 24 00:58:58 2001
+++ setup.S     Thu Oct 18 11:36:24 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 <[e-mail]>
  *
+ * 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?
+ * <[e-mail]> october 2001
  */
 
 #include <linux/config.h>
@@ -297,13 +301,13 @@
 
 #define SMAP  0x534d4150
 
-meme820:
+mem_e820:
         xorl    %ebx, %ebx                      # continuation counter
         movw    $E820MAP, %di                   # point into the whitelist
                                                 # so we can have the bios
                                                 # directly write into it.
 
-jmpe820:
+jmp820:
         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
 again820:
-        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
 bail820:
 
 
@@ -344,10 +346,10 @@
 # alternative new memory detection scheme, and it's sensible
 # to write everything into the same place.)
 
-meme801:
+mem_e801:
         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...
 
-e801usecxdx:
-        andl    $0xffff, %edx                   # clear sign extend
+e801use_cxdx:
+        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.
 mem88:
 
 #endif
+        #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)
+mem_cmos:
+        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
+        
+mem_store:
+        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
+mem_store2:
         movw    %ax, (2)
 
 # Set the keyboard repeat rate to the max