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MBR Bootloader: dynamically determine boot loader memory segment instead of hardcoded values (proposed by neos6464 at https://sourceforge.net/p/veracrypt/tickets/240/)

This commit is contained in:
Mounir IDRASSI 2018-12-02 06:02:42 +01:00
parent e0049f601a
commit 3e1460b618
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GPG Key ID: 02C30AE90FAE4A6F
1 changed files with 15 additions and 14 deletions
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29
src/Boot/Windows/BootSector.asm
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@ -43,23 +43,24 @@ main:
call print
skip_loader_name_msg:
; Determine boot loader segment
mov ax, TC_BOOT_LOADER_SEGMENT
; Determine boot loader segment
mov ax, word ptr [ds:413h] ;available kB from BIOS
sub ax, TC_BOOT_MEMORY_REQUIRED ;minus TC_BOOT_MEMORY_REQUIRED
jc mem_toolow
and ax, 0FFE0h ;32K align
shl ax, 6 ;convert kB to segment addr (*1024/16)
cmp ax, 8000h
jb mem_toolow ;we can't load below 8000h
cmp ax, TC_BOOT_LOADER_SEGMENT
jbe memory_ok ;don't load above TC_BOOT_LOADER_SEGMENT (9000h)
mov ax, TC_BOOT_LOADER_SEGMENT
jmp memory_ok
; Check available memory
cmp word ptr [ds:413h], TC_BOOT_LOADER_SEGMENT / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
jge memory_ok
mov ax, TC_BOOT_LOADER_SEGMENT_LOW
cmp word ptr [ds:413h], TC_BOOT_LOADER_SEGMENT_LOW / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
jge memory_ok
; Insufficient memory
mov ax, TC_BOOT_LOADER_LOWMEM_SEGMENT
mem_toolow:
mov ax, TC_BOOT_LOADER_LOWMEM_SEGMENT
memory_ok:
mov es, ax
mov es, ax
; Clear BSS section
xor al, al