source: bootcd/isolinux/syslinux-6.03/core/layout.inc

; -----------------------------------------------------------------------
;
;   Copyright 1994-2009 H. Peter Anvin - All Rights Reserved
;   Copyright 2009-2010 Intel Corporation; author: H. Peter Anvin
;
;   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,
;   Bostom MA 02111-1307, USA; either version 2 of the License, or
;   (at your option) any later version; incorporated herein by reference.
;
; -----------------------------------------------------------------------

;
; layout.inc
;
; Memory layout of segments
;

                ; Default to 16-bit code
                bits 16

; Memory below 0800h is reserved for the BIOS and the MBR.
BSS_START       equ 0800h

; Text starts at the load address of 07C00h.
TEXT_START      equ 7C00h

;
; 16-bit stack layout
;
; PXELINUX: There are apparently some AMI BIOSes in the field which
; put their BEV stack somewhere below 7C00h (and therefore don't
; handle localboot properly), so avoid that immediate memory region.
; The range that is known to be bad is approximately 75E8..7C00; the
; lower bound is tight.
; 
                global STACK_LEN, STACK_TOP, STACK_BASE
STACK_LEN       equ 4096
%if IS_PXELINUX
STACK_TOP       equ 7000h
%else
STACK_TOP       equ 7c00h
%endif
STACK_BASE      equ STACK_TOP - STACK_LEN

; The secondary BSS section, above the text; we really wish we could
; just make it follow .bcopy32 or hang off the end,
; but it doesn't seem to work that way.
LATEBSS_START   equ 0B800h

;
; 32-bit stack layout
;
STACK32_LEN     equ 64*1024

                section .stack          nobits write align=4096
                resb STACK32_LEN

;
; The various sections and their relationship
;
                ; Use .earlybss for things that MUST be in low memory.
                section .earlybss       nobits write
                section .config         write progbits align=4
                section .replacestub    exec write progbits align=16
                section .gentextnr      exec write nobits align=16
                section .stack16        write nobits align=16

                ; Use .bss16 for things that doesn't have to be in low memory;
                ; .earlybss should be used for things that absolutely have
                ; to be below 0x7c00.
                section .bss16          write nobits align=16

%if 0 ; IS_PXELINUX
                ; Warning here: RBFG build 22 randomly overwrites
                ; memory location [0x5680,0x576c), possibly more.  It
                ; seems that it gets confused and screws up the
                ; pointer to its own internal packet buffer and starts
                ; writing a received ARP packet into low memory.
                section .rbfg           write nobits
RBFG_brainfuck: resb 2048               ; Bigger than an Ethernet packet...
%endif

                section .init           exec write progbits align=1
                section .text16         exec write progbits align=1
                section .textnr         exec nowrite progbits align=1
                section .bcopyxx.text   exec nowrite progbits align=16
                section .bcopyxx.data   noexec write progbits align=16
                section .data16         noexec write progbits align=16

                section .adv            write nobits align=512

                ; .uibss contains bss data which is guaranteed to be
                ; safe to clobber during the loading of the image.  This
                ; is because while loading the primary image we will clobber
                ; the spillover from the last fractional sector load.
                section .uibss          write nobits align=16

                section .savedata       write nobits align=16

                ; Symbols from linker script
%macro SECINFO 1
                extern __%1_start, __%1_lma, __%1_end
                extern __%1_len, __%1_dwords
%endmacro
                SECINFO bss16
                SECINFO uibss
                SECINFO config
                SECINFO replacestub
                SECINFO bcopyxx

                SECINFO pm_code
                SECINFO high_clear

                SECINFO bss

                extern free_high_memory

                global _start

                section .text16

;
; Segment assignments in the bottom 640K
; Keep the low-memory footprint as small as possible... overrun is a hard
; failure!
;

serial_buf_size equ 4096                ; Should be a power of 2

;
; Transfer buffer segment: guaranteed to be aligned 64K, used for disk I/O
; One symbol for the segment number, one for the absolute address
;
                extern  xfer_buf_seg
                section .xfer_buf       write nobits align=65536
                global  core_xfer_buf:data hidden
core_xfer_buf   resb 65536

;
; Segment for the real mode code (needed as long as we have a in-kernel
; loader and/or COM16 support.
; One symbol for the segment number, one for the absolute address
;
                extern  real_mode_seg
                section .real_mode      write nobits align=65536
                global  core_real_mode:data hidden
core_real_mode  resb 65536
comboot_seg     equ real_mode_seg       ; COMBOOT image loading zone

;
; At the very end, the lowmem heap
;
                extern __lowmem_heap
min_lowmem_heap equ 65536

                section .text16