我正在学习如何使用汇编语言(NASM)手动构建头文件并将文件组装成二进制文件以创建Dos .EXE文件。我在页面选项方面遇到了问题(包括总页数和最后一页的字节计数)。无论我将初始值设置得多小,程序都能正常工作。
作为一个极端案例,即使设置1个字节的1页,以下程序也能正常运行:
;
; the smallest possible "Hello, World!" .EXE (DOS MZ) file
; assemble with:
; nasm -f bin -w+all -O0 smallest_hello_exe.asm -o ASM.EXE
;
bits 16
cpu 8086
;
; by setting cs:ip=-10h:100h instead of 0h:0h inside the .EXE header
; (identical assignments), we achieve the following two advantages:
; 1) ds==cs, so no "push cs pop ds" is needed in order for ds:dx
; to point to the message string
; 2) we can exit by int 20h instead of int 21h, thus omitting the
; ah=4ch assignment
; (int 20h requires that cs points to the PSP segment)
;
;
; we do not the address calculations to take the .EXE header into account
; so we must subtract its length (20h) by an "org -20h"
; but, since ip will be 100h, we must also issue an "org 100h"
; and, since 0x100-0x20=0xE0...
org 0xE0 ; 100h for ip value - 20h for header
section .text align=1
;
; the MZ .EXE header structure
; 28 bytes long
; 1 pararaph equals 16 bytes
; 1 page equals 512 bytes
; suggested reading: int 21h,ah=4bh procedure
;
host_exe_header:
.signature: dw 'MZ' ; the 'MZ' characters
.last_page_size: dw 1 ; number of used bytes in the final file page, 0 for all
.page_count: dw 1 ; number of file pages including any last partial page
.reloc: dw 0 ; number of relocation entries after the header
.paragraphs: dw 2 ; size of header + relocation table, in paragraphs
.minalloc: dw 0 ; minimum required additional memory, in paragraphs
.maxalloc: dw 0xFFFF ; maximum memory to be allocated, in paragraphs
.in_ss: dw 0 ; initial relative value of the stack segment
.in_sp: dw 0xF000 ; initial sp value
.checksum: dw 0 ; checksum: 1's complement of sum of all words
.in_ip: dw 100h ; initial ip value
.in_cs: dw -10h ; initial relative value of the text segment
.offset: dw 0 ; offset of the relocation table from start of header
.overlay: dw 0 ; overlay value (0h = main program)
; pad header (its size in bytes must be a multiple of 16)
times (32-$+$$) db 0
mov dx,message
mov ah,09h ; write string ds:dx to stdout
int 21h
int 20h
section .data align=1
message: db 'Hello, World!$'
section .bss align=1
通过尝试不同大小的程序,我得出结论,Dos会将每一页的512个字节全部加载到内存中。如果是这样,那么最后一页的字节数有何意义?
它是否会干扰.bss、堆栈数据和/或动态内存分配?