我正在尝试从文件中间读取数据。
我使用int 21h,啊= 3fh,但它从头开始读取文件。
有没有办法不从文件的开头读取数据?
(ASM 8086,如果相关,我正在使用TASM)
答案 0 :(得分:1)
是的,您可以使用int 21h功能42h - 打开文件后移动文件指针。
mov ah,42h ;function
mov al,0 ;to calculate offset from beginning of file
mov bx,handle ;from opening the file
mov cx,yyyy ;most significant part of offset
mov dx,xxxx ;least significant part of offset
int 21h ;system call
jc error ;check if errro
文件的下一次读取将从此位置开始。
al
0 = offset from beginning of file
1 = offset from current position (cx:dx is signed)
2 = offset from end of file (ditto)
我建议您现在找到我给您提示的文档。
答案 1 :(得分:1)
接下来是一个完整的程序来完成你想要实现的目标:它打开文件,跳转到中间位置(字节500),读取内容,然后关闭文件。这是用EMU8086制作的:
.model small
.stack 100h
.data
filename db 'tree_img.png',0
filehandler dw ?
buffer db 10 dup (?)
.code
start:
;INITIALIZE DATA SEGMENT.
mov ax, @data
mov ds, ax
call read_middle ;<==============
;WAIT FOR ANY KEY.
mov ah, 7
int 21h
;FINISH PROGRAM.
mov ax, 4c00h
int 21h
;-----------------------------------------
read_middle proc
;OPEN FILE.
mov ah, 3dh ;SERVICE TO OPEN FILE.
mov al, 0 ;OPEN AS READ ONLY.
lea dx, filename
int 21h
mov filehandler, ax ;NECESSARY FOR OPERATIONS ON FILE.
;JUMP TO POSITION INSIDE THE FILE. <==============
mov ah, 42h ;SERVICE FOR SEEK.
mov al, 0 ;START FROM THE BEGINNING OF FILE.
mov bx, filehandler ;FILE.
mov cx, 0 ;THE FILE POSITION MUST BE PLACED IN
mov dx, 500 ;CX:DX, EXAMPLE, TO JUMP TO POSITION
int 21h ;14000000 SET CX:DX = D5:9F80.
;READ ONE CHAR FROM CURRENT FILE POSITION.
mov ah, 3fh ;SERVICE TO READ FROM FILE.
mov bx, filehandler
mov cx, 1 ;HOW MANY BYTES TO READ.
lea dx, buffer ;WHERE TO STORE THE READ BYTES.
int 21h
;CLOSE FILE.
mov ah, 3eh ;SERVICE TO CLOSE FILE.
mov bx, filehandler
int 21h
ret
read_middle endp
;-----------------------------------------
end start
答案 2 :(得分:0)
要获取DI中的文件长度:SI
xor cx, cx ; set high
xor dx, dx ; set low
mov ax, 4202h ; Get length of file from end
int 21h
jc ERROR
mov si, ax ; return low
mov di, dx ; return high
xor cx, cx ; set high
xor dx, dx ; set low
mov ax, 4200h ; Set file pointer to the start position
int 21h
jc ERROR
RBIL-&GT; inter61b.zip-&GT; INTERRUP.F
http://www.cs.cmu.edu/~ralf/files.html
--------D-2142-------------------------------
INT 21 - DOS 2+ - "LSEEK" - SET CURRENT FILE POSITION
AH = 42h
AL = origin of move
00h start of file
01h current file position
02h end of file
BX = file handle
CX:DX = (signed) offset from origin of new file position
Return: CF clear if successful
DX:AX = new file position in bytes from start of file
CF set on error
AX = error code (01h,06h) (see #01680 at AH=59h/BX=0000h)
Notes: for origins 01h and 02h, the pointer may be positioned before the
start of the file; no error is returned in that case (except under
Windows NT), but subsequent attempts at I/O will produce errors
if the new position is beyond the current end of file, the file will
be extended by the next write (see AH=40h); for FAT32 drives, the
file must have been opened with AX=6C00h with the "extended size"
flag in order to expand the file beyond 2GB
BUG: using this method to grow a file from zero bytes to a very large size
can corrupt the FAT in some versions of DOS; the file should first be
grown from zero to one byte and then to the desired large size
SeeAlso: AH=24h,INT 2F/AX=1228h