32位汇编语言学习笔记(29)--在NASM中使用宏
宏的语法规则与汇编指令没有关系，只是为了管理代码复杂度而设计的类似于高级语言的特性。NASM的宏语法格式如下：%macro 宏名称 参数个数宏的内容%endmacro宏的第一个参数用%1表示，第二个参数用%2表示，依次类推。《Assembly Language step by step programming with linux》在第10章给出了一个使用宏版本的光标控
宏的语法规则与汇编指令没有关系，只是为了管理代码复杂度而设计的类似于高级语言的特性。NASM的宏语法格式如下：
%macro 宏名称 参数个数
宏的内容
%endmacro
宏的第一个参数用%1表示，第二个参数用%2表示，依次类推。
《Assembly Language step by step programming with linux》在第10章给出了一个使用宏版本的光标控制程序：
; Executable name : eatmacro
; Version : 1.0
; Created date : 2/21/2009
; Last update : 2/23/2009
; Author : Jeff Duntemann
; Description : A simple program in assembly for Linux, using
; : NASM 2.05, demonstrating the use of escape
; : sequences to do simple "full-screen" text output
; ; through macros rather than procedures
;
; Build using these commands:
; nasm -f elf -g -F stabs eatmacro.asm
; ld -o eatmacro eatmacro.o
;
;
section .data ; Section containing initialised data
SCRWIDTH: equ 80 ; By default we assume 80 chars wide
PosTerm: db 27,"[01;01H" ; <ESC>[<Y>;<X>H
POSLEN: equ $-PosTerm ; Length of term position string
ClearTerm: db 27,"[2J" ; <ESC>[2J
CLEARLEN equ $-ClearTerm ; Length of term clear string
AdMsg: db "Eat At Joe's!" ; Ad message
ADLEN: equ $-AdMsg ; Length of ad message
Prompt: db "Press Enter: " ; User prompt
PROMPTLEN: equ $-Prompt ; Length of user prompt
; This table gives us pairs of ASCII digits from 0-80. Rather than
; calculate ASCII digits to insert in the terminal control string,
; we look them up in the table and read back two digits at once to
; a 16-bit register like DX, which we then poke into the terminal
; control string PosTerm at the appropriate place. See GotoXY.
; If you intend to work on a larger console than 80 X 80, you must
; add additional ASCII digit encoding to the end of Digits. Keep in
; mind that the code shown here will only work up to 99 X 99.
Digits: db "0001020304050607080910111213141516171819"
db "2021222324252627282930313233343536373839"
db "4041424344454647484950515253545556575859"
db "606162636465666768697071727374757677787980"
SECTION .bss ; Section containing uninitialized data
SECTION .text ; Section containing code
;-------------------------------------------------------------------------
; ExitProg: Terminate program and return to Linux
; UPDATED: 4/23/2009
; IN: Nothing
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_exit
; DESCRIPTION: Calls sys_edit to terminate the program and return
; control to Linux
%macro ExitProg 0
mov eax,1 ; Code for Exit Syscall
mov ebx,0 ; Return a code of zero
int 80H ; Make kernel call
%endmacro
;-------------------------------------------------------------------------
; WaitEnter: Wait for the user to press Enter at the console
; UPDATED: 4/23/2009
; IN: Nothing
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_read
; DESCRIPTION: Calls sys_read to wait for the user to type a newline at
; the console
%macro WaitEnter 0
mov eax,3 ; Code for sys_read
mov ebx,0 ; Specify File Descriptor 0: Stdin
int 80H ; Make kernel call
%endmacro
;-------------------------------------------------------------------------
; WriteStr: Send a string to the Linux console
; UPDATED: 4/21/2009
; IN: String address in %1, string length in %2
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_write
; DESCRIPTION: Displays a string to the Linux console through a
; sys_write kernel call
%macro WriteStr 2 ; %1 = String address; %2 = string length
push eax ; Save pertinent registers
push ebx
mov ecx,%1 ; Put string address into ECX
mov edx,%2 ; Put string length into EDX
mov eax,4 ; Specify sys_write call
mov ebx,1 ; Specify File Descriptor 1: Stdout
int 80H ; Make the kernel call
pop ebx ; Restore pertinent registers
pop eax
%endmacro
;-------------------------------------------------------------------------
; ClrScr: Clear the Linux console
; UPDATED: 4/23/2009
; IN: Nothing
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_write
; DESCRIPTION: Sends the predefined control string <ESC>[2J to the
; console, which clears the full display
%macro ClrScr 0
push eax ; Save pertinent registers
push ebx
push ecx
push edx
; Use WriteStr macro to write control string to console:
WriteStr ClearTerm,CLEARLEN
pop edx ; Restore pertinent registers
pop ecx
pop ebx
pop eax
%endmacro
;-------------------------------------------------------------------------
; GotoXY: Position the Linux Console cursor to an X,Y position
; UPDATED: 4/23/2009
; IN: X in %1, Y in %2
; RETURNS: Nothing
; MODIFIES: PosTerm terminal control sequence string
; CALLS: Kernel sys_write
; DESCRIPTION: Prepares a terminal control string for the X,Y coordinates
; passed in AL and AH and calls sys_write to position the
; console cursor to that X,Y position. Writing text to the
; console after calling GotoXY will begin display of text
; at that X,Y position.
%macro GotoXY 2 ; %1 is X value; %2 id Y value
pushad ; Save caller's registers
xor edx,edx ; Zero EDX
xor ecx,ecx ; Ditto ECX
; Poke the Y digits:
mov dl,%2 ; Put Y value into offset term EDX
mov cx,word [Digits+edx*2] ; Fetch decimal digits to CX
mov word [PosTerm+2],cx ; Poke digits into control string
; Poke the X digits:
mov dl,%1 ; Put X value into offset term EDX
mov cx,word [Digits+edx*2] ; Fetch decimal digits to CX
mov word [PosTerm+5],cx ; Poke digits into control string
; Send control sequence to stdout:
WriteStr PosTerm,POSLEN
; Wrap up and go home:
popad ; Restore caller's registers
%endmacro
;-------------------------------------------------------------------------
; WriteCtr: Send a string centered to an 80-char wide Linux console
; UPDATED: 4/23/2009
; IN: Y value in %1, String address in %2, string length in %3
; RETURNS: Nothing
; MODIFIES: PosTerm terminal control sequence string
; CALLS: GotoXY, WriteStr
; DESCRIPTION: Displays a string to the Linux console centered in an
; 80-column display. Calculates the X for the passed-in
; string length, then calls GotoXY and WriteStr to send
; the string to the console
%macro WriteCtr 3 ; %1 = row; %2 = String addr; %3 = String length
push ebx ; Save caller's EBX
push edx ; Save caller's EDX
mov edx,%3 ; Load string length into EDX
xor ebx,ebx ; Zero EBX
mov bl,SCRWIDTH ; Load the screen width value to BL
sub bl,dl ; Calc diff. of screen width and string length
shr bl,1 ; Divide difference by two for X value
GotoXY bl,%1 ; Position the cursor for display
WriteStr %2,%3 ; Write the string to the console
pop edx ; Restore caller's EDX
pop ebx ; Restore caller's EBX
%endmacro
global _start ; Linker needs this to find the entry point!
_start:
nop ; This no-op keeps gdb happy...
; First we clear the terminal display...
ClrScr
; Then we post the ad message centered on the 80-wide console:
WriteCtr 12,AdMsg,ADLEN
; Position the cursor for the "Press Enter" prompt:
GotoXY 1,23
; Display the "Press Enter" prompt:
WriteStr Prompt,PROMPTLEN
; Wait for the user to press Enter:
WaitEnter
; ...and we're done!
ExitProgmakefile文件内容如下：
eatmacro: eatmacro.o
ld -o eatmacro eatmacro.o
eatmacro.o: eatmacro.asm
nasm -f elf -l eatmacro.lst -g -F stabs eatmacro.asm程序逻辑与第一版光标控制程序是一样的，执行结果也相同。下面分析一下使用宏版本的反汇编的代码，只分析一小段，其余的都类似：
[root@bogon eatmacro]# objdump -d eatmacro
eatmacro: file format elf32-i386
Disassembly of section .text:
08048080 <_start>:
8048080: 90 nop
8048081: 50 push %eax // ClrScr宏展开
8048082: 53 push %ebx
8048083: 51 push %ecx
8048084: 52 push %edx
8048085: 50 push %eax // WriteStr ClearTerm,CLEARLEN宏展开是在这里
8048086: 53 push %ebx
8048087: b9 8c 91 04 08 mov $0x804918c,%ecx //$0x804918c是ClearTerm字节数组的地址
804808c: ba 04 00 00 00 mov $0x4,%edx //$0x4是CLEARLEN
8048091: b8 04 00 00 00 mov $0x4,%eax
8048096: bb 01 00 00 00 mov $0x1,%ebx
804809b: cd 80 int $0x80
804809d: 5b pop %ebx
804809e: 58 pop %eax // WriteStr宏在这里结束
804809f: 5a pop %edx
80480a0: 59 pop %ecx
80480a1: 5b pop %ebx
80480a2: 58 pop %eax // ClrScr宏结束
……
因此可以看出，NASM就是在预处理时，把宏展开，简单的进行了的代码替换。
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