root/core/highmem.inc

;; -----------------------------------------------------------------------
;;
;;   Copyright 1994-2008 H. Peter Anvin - All Rights Reserved
;;
;;   This program is free software; you can redistribute it and/or modify
;;   it under the terms of the GNU General Public License as published by
;;   the Free Software Foundation, Inc., 53 Temple Place Ste 330,
;;   Boston MA 02111-1307, USA; either version 2 of the License, or
;;   (at your option) any later version; incorporated herein by reference.
;;
;; -----------------------------------------------------------------------

;;
;; highmem.inc
;;
;; Probe for the size of high memory.  This can be overridden by a
;; mem= command on the command line while booting a new kernel.
;;

                section .text16

;
; This is set up as a subroutine; it will set up the global variable
; HighMemSize.  All registers are preserved.
;
highmemsize:
                push es
                pushfd
                pushad

                push cs
                pop es

;
; First, try INT 15:E820 (get BIOS memory map)
;
; Note: we may have to scan this multiple times, because some (daft) BIOSes
; report main memory as multiple contiguous ranges...
;
get_e820:
                mov dword [E820Max],-(1 << 20)  ; Max amount of high memory
                mov dword [E820Mem],(1 << 20)   ; End of detected high memory
.start_over:
                mov di,E820Buf
                xor ax,ax
                mov cx,10
                rep stosw                       ; Clear buffer
                xor ebx,ebx                     ; Start with first record
                jmp short .do_e820              ; Skip "at end" check first time!
.int_loop:      and ebx,ebx                     ; If we're back at beginning...
                jz .e820_done                   ; ... we're done
.do_e820:       mov eax,0000E820h
                mov edx,534D4150h               ; "SMAP" backwards
                xor ecx,ecx
                mov cl,20                       ; ECX <- 20 (size of buffer)
                mov di,E820Buf
                int 15h
                jnc .no_carry
                ; If carry, ebx == 0 means error, ebx != 0 means we're done
                and ebx,ebx
                jnz .e820_done
                jmp no_e820
.no_carry:
                cmp eax,534D4150h
                jne no_e820
                cmp cx,20
                jb no_e820

;
; Look for a memory block starting at <= 1 MB and continuing upward
;
                cmp dword [E820Buf+4], byte 0
                ja .int_loop                    ; Start >= 4 GB?
                mov eax, [E820Buf]
                cmp dword [E820Buf+16],1
                je .is_ram                      ; Is it memory?
                ;
                ; Non-memory range.  Remember this as a limit; some BIOSes get the length
                ; of primary RAM incorrect!
                ;
.not_ram:
                cmp eax, (1 << 20)
                jb .int_loop                    ; Starts in lowmem region
                cmp eax,[E820Max]
                jae .int_loop                   ; Already above limit
                mov [E820Max],eax               ; Set limit
                jmp .int_loop

.is_ram:
                cmp eax,[E820Mem]
                ja .int_loop                    ; Not contiguous with our starting point
                add eax,[E820Buf+8]
                jc .overflow
                cmp dword [E820Buf+12],0
                je .nooverflow
.overflow:
                or eax,-1
.nooverflow:
                cmp eax,[E820Mem]
                jbe .int_loop                   ; All is below our baseline
                mov [E820Mem],eax
                jmp .start_over                 ; Start over in case we find an adjacent range

.e820_done:
                mov eax,[E820Mem]
                cmp eax,[E820Max]
                jna .not_limited
                mov eax,[E820Max]
.not_limited:
                cmp eax,(1 << 20)
                ja got_highmem                  ; Did we actually find memory?
                ; otherwise fall through

;
; INT 15:E820 failed.  Try INT 15:E801.
;
no_e820:
                mov ax,0e801h                   ; Query high memory (semi-recent)
                int 15h
                jc no_e801
                cmp ax,3c00h
                ja no_e801                      ; > 3C00h something's wrong with this call
                jb e801_hole                    ; If memory hole we can only use low part

                mov ax,bx
                shl eax,16                      ; 64K chunks
                add eax,(16 << 20)              ; Add first 16M
                jmp short got_highmem

;
; INT 15:E801 failed.  Try INT 15:88.
;
no_e801:
                mov ah,88h                      ; Query high memory (oldest)
                int 15h
                cmp ax,14*1024                  ; Don't trust memory >15M
                jna e801_hole
                mov ax,14*1024
e801_hole:
                and eax,0ffffh
                shl eax,10                      ; Convert from kilobytes
                add eax,(1 << 20)               ; First megabyte
got_highmem:
%if HIGHMEM_SLOP != 0
                sub eax,HIGHMEM_SLOP
%endif
                mov [HighMemSize],eax
                popad
                popfd
                pop es
                ret                             ; Done!

                section .bss16
                alignb 4
E820Buf         resd 5                  ; INT 15:E820 data buffer
E820Mem         resd 1                  ; Memory detected by E820
E820Max         resd 1                  ; Is E820 memory capped?
; HighMemSize is defined in com32.inc