32位汇编语言学习笔记(42)--测试时间相关函数


此程序出自《Assembly Language step by step programming with linux》第12章，用于演示时间相关的libc库函数的使用，程序中使用了如下的libc库函数：

time_t time( time_t *timer );

char *ctime( const time_t *timer );

double difftime( time_t timer1, time_t timer0 );

struct tm *localtime( const time_t *timer );

int getchar( void );

程序如下：

[SECTION .data]			; Section containing initialised data

TimeMsg  db "Hey, what time is it?  It's %s",10,0
YrMsg	 db "The year is %d.",10,0
Elapsed  db "A total of %d seconds has elapsed since program began running.",10,0

[SECTION .bss]			; Section containing uninitialized data

OldTime	 resd 1			; Reserve 3 integers (doubles) for time values
NewTime  resd 1
TimeDiff resd 1
TimeStr  resb 40		; Reserve 40 bytes for time string
TmCopy	 resd 9			; Reserve 9 integer fields for time struct tm
[SECTION .text]			; Section containing code

extern ctime
extern difftime
extern getchar
extern printf
extern localtime
extern time

global main			; Required so linker can find entry point

main:
push ebp		; Set up stack frame for debugger
mov ebp,esp
push ebx		; Program must preserve EBP, EBX, ESI, & EDI
push esi
push edi
;;; Everything before this is boilerplate; use it for all ordinary apps!

; Generate a time_t calendar time value with clib's time function
push 0			; Push a 32-bit null pointer to stack,
;   since we don't need a buffer.
call time		; Returns calendar time in EAX
add esp,4		; Clean up stack after call
mov [OldTime],eax	; Save time value in memory variable

; Generate a string summary of local time with clib's ctime function
push OldTime		; Push address of calendar time value
call ctime		; Returns pointer to ASCII time string in EAX
add esp,4		; Stack cleanup for 1 parm

push eax		; Push pointer to ASCII time string on stack
push TimeMsg		; Push pointer to base message text string
call printf		; Merge and display the two strings
add esp,8		; Stack cleanup: 2 parms X 4 bytes = 8

; Generate local time values into clib's static tm struct
push dword OldTime	; Push address of calendar time value
call localtime		; Returns pointer to static time structure in EAX
add esp,4		; Stack cleanup for 1 parm

; Make a local copy of clib's static tm struct
mov esi,eax		; Copy address of static tm from eax to ESI
mov edi,TmCopy		; Put the address of the local tm copy in EDI
mov ecx,9		; A tm struct is 9 dwords in size under Linux
cld			; Clear DF so we move up-memory
rep movsd		; Copy static tm struct to local copy

; Display one of the fields in the tm structure
mov edx,dword [TmCopy+20] ; Year field is 20 bytes offset into tm
add edx,1900		; Year field is # of years since 1900
push edx		; Push value onto the stack
push YrMsg		; Push address of the base string
call printf		; Display string and year value with printf
add esp,8		; Stack cleanup: 2 parms X 4 bytes = 8

; Wait a few seconds for user to press Enter so we have a time difference:
call getchar		; Wait for user to press Enter

; Calculating seconds passed since program began running:
push dword 0		; Push null ptr; we'll take value in EAX
call time		; Get current time value; return in EAX
add esp,4		; Clean up the stack
mov [NewTime],eax	; Save new time value

sub eax,[OldTime]	; Calculate time difference value
mov [TimeDiff],eax	; Save time difference value

push dword [TimeDiff]	; Push difference in seconds onto the stack
push Elapsed		; Push addr. of elapsed time message string
call printf		; Display elapsed time
add esp,8		; Stack cleanup for 1 parm

;;; Everything after this is boilerplate; use it for all ordinary apps!
pop edi			; Restore saved registers
pop esi
pop ebx
mov esp,ebp		; Destroy stack frame before returning
pop ebp
ret			; Return control to Linux

程序分析：

push 0 //函数参数timer=NULL

call time //调用time函数，返回当前时间，用与1970年1月1日0点0分0秒的相差的秒数表示。

add esp,4 //清理栈

mov [OldTime],eax //保存返回值到OldTime变量

push OldTime // 函数参数timer= OldTime

call ctime //调用ctime函数，返回时间信息字符串

add esp,4 //清理栈

push eax //时间字符串压入堆栈

push TimeMsg //时间显示格式

call printf //调用printf函数，打印时间信息

add esp,8 //清理栈，因为两个参数，所以加8

push dword OldTime //函数参数 timer= OldTime

call localtime //调用localtime函数，对时间值进行转换，返回tm结构体的指针。

add esp,4 //清理栈

mov esi,eax //esi是源，保存返回的tm结构体的地址

mov edi,TmCopy //edi=TmCopy，指定拷贝的目的地址

mov ecx,9 //ecx用于循环计数，tm结构体是9个dword，因此ecx=9

cld //清除DF标志，控制地址变化方向从低到高

rep movsd //进行拷贝，把返回的tm结构体拷贝到TmCopy

mov edx,dword [TmCopy+20] //edx=TmCopy的year字段

add edx,1900 //因为tm保存的是与1900年的差值，因此要加1900

push edx //把年份参数压入堆栈

push YrMsg //格式化字符串

call printf //调用printf函数打印年份信息

add esp,8 //清理栈

call getchar //等待用户键入回车键

push dword 0 //函数参数timer=NULL

call time //调用time函数，返回当前时间。

add esp,4 //清理栈

mov [NewTime],eax //把当前时间值保存到NewTime中。

sub eax,[OldTime] //用NewTime- OldTime的值放入eax

mov [TimeDiff],eax //差值保存到TimeDiff中

push dword [TimeDiff] //把差值作为函数参数，压入堆栈

push Elapsed //格式化字符串压入堆栈

call printf //调用printf函数，显示消耗的秒数信息。

add esp,8 //清理栈

makefile文件内容：

timetest: timetest.o
gcc timetest.o -o timetest
timetest.o: timetest.asm
nasm -f elf -g -F stabs timetest.asm

测试：

[root@bogon timetest]# make
nasm -f elf -g -F stabs timetest.asm
gcc timetest.o -o timetest
[root@bogon timetest]# ./timetest
Hey, what time is it?  It's Wed Jan 14 15:12:39 2015

The year is 2015.

A total of 2 seconds has elapsed since program began running.