new assembly language for intel-based computers (fourth edition) cch14

Chapter 14 Disks Fundamentals,Every operating system has some type of disk management system. At the lowest level, it manages partitions. At the nest-highest level, it manages files and directories. A file system must keep track of the location, sizes, and attributes of each disk file. A cluster is the smallest unit space used by a file; it consists of one or more adjacent disk sectors. A file system stores each file as a linked sequence of clusters. Actually, a cluster size of 4,096 (4 KB) is considered an efficient way to store small files. A chain of clusters is referenced by a file allocation table (FAT) that keep track of all clusters used by a file. A pointer to the first cluster entry in the FAT is stored in each files directory entry.,2. MS-DOS directory StructureEach MS-DOS directory entry is 32 bytes long and contains the fields.,AttributeThe attribute field identifies the type of field. The field is bit-mapped and usually contains a combination of one of the values. The two reserved bits should always be 0. The archive bit is set when a file is modified. The subdirectory bit is set if the entry contains the name of a subdirectory. The volume label identifies the entry as the name of a disk volume. The system file bit indicates that file is part of the operating system. The hidden file bits makes the file hidden; its name does not appear in d display of the directory. The read-only bit prevents the file from being deleted or modified in any way. Finally, an attribute value of 0Fh indicates the current directory is for an extended filename.,File Attribute Byte Fields,Function Call of File Management,File/Device HandlesBoth MS-DOS and MS-Windows use 16-bit integers called handles to identify files and I/O devices There are five predefined device.0=Keyboard (standard input)1=Console (standard output)2=Error output3=Auxiliary device (asynchronous)4=Printer,Each I/O function has a common characteristic: if it fails, the Carry flag is set and an error code is returned in AX.,Example Modifying the Attribute of the File (43H)MOV AH, 43H ;check or change attributeMOV AL,01 ;set file attributeMOV CX,01 ;read onlyMOV DX, SEG FNAME ;addr of ASCIZ stringMOV DS, DXMOV DX,OFFSET FNAMEINT 21H ; call DOS,Example Write disk filePATHNM1 DB E:ACCOUNTS.FIL, 00HHANDLE1 DW ? ;file handle MOV AH, 3CH ; request creat MOV CX, 00 ; normal file LEA DX, PATHNM1 ; ASCIZ string INT 21H ; call DOS JC ERROR ; exit if error MOV HANDLE1, AX ; save handle in DW,HANDLE1 DW ?OUTREC DB 265 DUP(?) MOV AH, 40H ; request write MOV BX, HANDLE1 ; handle MOV CX, 265 ; number of bytes LEA DX, OUTREC ; address of output area INT 21H ; call interrupt service JC ERROR2 ; special action if error CMP AX, 265 ; all bytes written? JNE ERROR3 ; if not, error,MOV AH, 3EH ;request closeMOV BX, HANDLE1 ;file handleINT 21H ;call DOS,LEA DX, OUTREC ; addr of output area INT 21H ; call interrupt service JC ERROR2 ; special action if error CMP AX, 265 ; all bytes written? JNE ERROR3 ; if not, error,ExampleCreating a File,.model small .stack 64 .datanamepar label bytemaxlen db 30namelen db ?namerec db 30 dup( ), odh, oahclrf db 13, 10, $,errcde db 0handle dw ?pathname db D:NAME.DAT,0prompt db name ?Row db 01opnmsg db * open error *,0dh,0ah,$wrtmsg db * write error *,oah,odh,$ .codebegin proc far move ax,data,mov ds, ax mov es, ax mov ax, 0600h call scren ; clear screen call curs ; set cursor call creath ; create file cmp errcde, 0 ; create error? jz contin ; yes, continuecontin: call proach cmp namelen, 0 ;end of inpout,jne contin ; no, continue call clseh ; yes, closeback: mov ax, 4c00h ; return to DOS int 21hbegin endp;-; create disk filecreath proc near mov ah, 3ch ; request create mov cx, 0 ; normal attribute lea dx, pathname,int 21h jc al ; error? mov handle, ax ; no, save handle retal: lea dx, opnmsg call errm retcreath endp;-; accept inputproch proc near,mov ah, 40h ; request display mov bx, 01 ; 01=output display mov cx, 06 ; length of promt lea dx, prompt ; display prompt int 21h mov ah, 0ah ; request input lea dx, namepar ; accept name int 21h cmp namelen, 0 jne b1; ;yes,ret ; no, exitb1: mov al, 20h ; blank for storing sub ch, ch mov cl, namelen ; length lea di, namerce add di, cx ; calculate+length neg cx ; calculate remaining add cx, 30 ; length cld rep stosb ; set to blank,call writh ; write disk record call scrl ; check for scroll retproch endp;-; check for scrollscrl proc near cmp row, 18h ; bottom of screen? jae cl ; yes, bypass mov ah, 09 ; no, to next row lea dx, clrf,mov ah, 09 ; no, to next row lea dx, clrf int 21h inc row ; no, add to row retc1: mov ax, 0601h ;scroll one row call scren call curs ; reset cursor retscrl endp,; write disk recordwrith proc near mov ah, 40h ; request write mov bx, handle mov cx, 32 ; for name and CR/LF lea dx, namerec int 21h jnc dl ; valid writ? lea dx, writmsg ; no call errm ; call error rotine,mov namelen, 0d1: retwrith endp;-; close disk fileclseh proc near mov namerec, 1ah ;set EOF mark call writh mov ah, 3eh ; request close mov bx, handle int 21h,clseh endp;-; scroll screenscreen proc near ; AX set on entry mov bh, 1eh ; set yellow on blue mov cx, 0 mov dx, 184fh int 10h ; scroll retscreen endp,; set cursorcurs proc near mov ah, 02 mov bh,0 mov dh, row ; set cursor int 10h retcurs endp;-; disk error routine,errm proc near ;DX contains address mov ah, 40h ; of message mov bx, 01 ; handle mov cx, 21 ; length int 21h mov errcde, 01 ; set error code reterrm endp;- end begin,Example Read disk file ; Read disk record created by hancreat .model small .stack 64 .dataendcde db 0handle dw ?ioarea db 32 dup( )pathnam db D:name.dat,0openms db * open error *,0dh,0ahreadmsg db * read error *, 0dh, 0ah,row db 0dseg ends;- .codebegin proc far mov ax, data mov ds, ax mov es, ax mov ax, 0600h call scren call curs,call openth cmp endcde, 0 jnzcontin: call readh cmp endcde, 0 jnz al call disp jmp continal: mov ax, 4c00h int 21hbeging endp,;-;open fileopenh proc near mov ah, 3dh ;request open mov al, 0 ; normal file lea dx, pathnma int 21h jc bl ; error mov handle, ax ; no, save handle ret,b1: mov endcde, 01 ;yes lea dx, openmsg ;display call errm ;error message retopenh endp;-; read disk recordreadh proc near,mov ah, 3fh ;request read mov bx, handle mov cx, 32 ;for name and CR/LF lea dx, ioarea int 21h jc c1 ;error on read? cmp ax, 0 ;end of file? je c2 cmp ioarea, 1ah ;EOF marker? je c2 ;yes, exit ret,c1: lea dx, readmsg ;no, invalid read call errmc2 mov endcde, 01 ;force end retreadh endp;-; display namedisph proc near mov ah, 40h ;request display mov bx, 01 ;set handle lea dx, ioarea,int 21h cmp row, 24 ;bottom of screen jae d1 inc row retd1: mov ax, 0601h call scren ;scroll call curs ; set cursor retdisph endp,;-;scroll screenscren proc near ; AX set on entry mov bh, 1eh ;set color mov cx, 0 mov dx, 184fh ;request scroll int 10h retscren endp,; set cursorcurs proc near mov ah, 2 ;request set cursor mov bh, 2 mov dh, row ;row mov dl,0 ;column int 10h retcurs endp;-;disk error routine,errm proc near mov ah, 40h ;DX contains address mov bx, 01 ; handle mov cx, 20 ;length of message int 21h reterrm endp;-end begin,Example Shift pointer of fileAL=00 absolute shift mode MOV AH, 42H ;request move pointer MOV AL, 00 ;to start of file MOV BX, HANDLE1 ;set file handle MOV CX, 00 ;high_order word of offset MOV DX, 1024 ;low_order word of offset INT 21H ;call DOS JC ERROR ;check error,AL=01 relative shift mode MOV BX, HANDLE ;file handle MOV CX, 0 ;high_order word MOV DX, N ;low_order word CMP DX, 0 ;if N0,JGE POINT ;yes NOT CX ;extending the signPOINT: MOV AL, 1 ;method1 MOV AH, 42H ;request move pointer INT 21H JC ERROR,3. AL=2 absolute inverse shift mode MOV AH, 42H ;request move pointer MOV AL, 2 ;method 2 MOV BX, HANDLE ;handle MOV CX, 0 ;offset MOV DX, 0 INT 21H ;call DOS JC ERROR ;check error,Example Random read fileTITLE READR.EXE-Read disk records randomly .model small .stack 64 .datahandle dw ? ;file handlerecindx dw ? ;record indexerrcde db 00 ;read error indicator,prompt db RECORD NUMBER?ioarea db 32 dup( ) ;disk record areapathnam db D:NAME.DAT,0openmsg db * open error *, 0dh, 0ahreadmsg db * read error *, 0dh, 0ahrow db 00col db 00recdpar label byte ;input parameter lists:maxlen db 3 ;maximum lengthactlen db ? ;actual lengthrecdno db 3 dup ( ) ;record number,.code .386main proc far mov ax, data ;intialize mov ds, ax ;sehment mov es, ax ;registers mov ax, 0600h call scrn ;clear screen call curs ;set cursor call openran ;open file call errcde ;valid open,cmp errcde, 0 ;valid open? jnz exit ;not, exitcallsub: call recnum ;request record # cmp actlen, 0 ;any more request? je exit ;no, exit call readran ;read disk record cmp errcde, 0 ;normal read? jnz next ;no, bypass call disp ;yes, display namenext: jmp callsub ;continueexit mov ax, 4c00h ;end processing int 21h,main endp;-;open fileopenran proc near mov ah, 3dh ;request open mov al, 0 ;normal file lea dx, pathnam int 21h jc errm1 ;error?,mov handle, ax ;no, save handl