Microprocessors (A) DOS Services

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1 1 Services

2 2 System Calls Operating System services: Disk and file system management Screen display and printing Keyboard entry Other I/O management Date and time Program run and terminate Command arguments

3 3 Calling System Functions Operating System calls: AH function code (identifies particular system call) Other parameters may be required in various registers Instruction INT 21 calls the function handler

4 4 Create File 1 Entry parameters: Function (AH): 3Ch DS:DX Address of zero terminated pathname CX File attribute The low order six bits of CX contain the following values: bit 0 File is a Read-Only file bit 1 File is a hidden file bit 2 File is a system file bit 3 File is a volume label name bit 4 File is a subdirectory bit 5 File has been archived 0100 JMP DB 'filename.txt' MOV DX, MOV CX, MOV AH,3C 011A INT 21

5 5 Exit parameters: Create File 2 If carry is set (CF=1), AX contains one of the following error codes: 3 Path not found 4 Too many open files 5 Access denied If the carry is clear (CF=0), AX contains the file handle value assigned by. Since this call creates a new file, opens the file for writing only.

6 6 Entry parameters: Function (AH): 3Dh AL file access value Open File 1 0 File opened for reading 1 File opened for writing 2 File opened for reading and writing DS:DX Point at a zero terminated string containing the filename.

7 7 Open File 2 Exit parameters: If carry is set (CF=1), AX contains one of the following error codes: 2 File not found 4 Too many open files 5 Access denied 12 Invalid access If the carry is clear (CF=0), AX contains the file handle value assigned by.

8 8 Close File Entry parameters: BX File Handle Function (AH): 3Eh Exit parameters: If carry flag is set, AX contains 6, the only possible error, an invalid handle error.

9 9 Read From a File Entry parameters Function (AH): 3Fh BX File handle CX Number of bytes to read DS:DX location of an array large enough to hold bytes read Exit parameters If carry is set, AX contains one of the following error codes 5 Access denied 6 Invalid handle If the carry flag is clear, AX contains the number of bytes read from the file Unless end of file (EOF) was reached, AX = CX If end of file was reached, AX < CX register This is the only test for the EOF condition

10 10 Write to a File Entry parameters Function (AH): 40h BX File handle CX Number of bytes to write DS:DX Address of buffer containing data to write Exit parameters If carry is set, AX contains one of the following error codes 5 Accessed denied 6 Invalid handle If carry is clear on return AX contains the number of bytes written to the file If AX < CX the disk is full and this should be treated as an error.

11 11 Function 42h File Read/Write Pointer (LSEEK) entry AH 42h AL method code 00h offset from beginning of file 01h offset from present location 02h offset from end of file BX file handle CX most significant half of offset DX least significant half of offset return AX low offset of new file pointer DX high offset of new file pointer CF 0 successful move CF 1 error and AX error code (1, 6) note 1) If pointer is at end of file, reflects file size in bytes. 2) The value in DX:AX is the absolute 32 bit byte offset from the beginning

12 12 Function 4Ch Terminate a Process (EXIT) Quit with ERRORLEVEL exit code entry AH 4Ch AL exit code in AL when called, if any, is passed to next process return none note 1) Control passes to or calling program 2) return code from AL can be retrieved by ERRORLEVEL or function 4Dh 3) all files opened by this process are closed, buffers are flushed, and 4) the disk directory is updated 5) Restores Terminate vector from PSP:000Ah 6) Ctrl-C vector from PSP:000Eh 7) Critical Error vector from PSP:0012h

13 13 INT 20h Terminate Program Ends program, updates, FAT, flushes buffers, restores registers Old program termination method not recommended by Microsoft 1) Program must place the segment address of the PSP control block in CS before calling this function. 2) All file buffers are flushed and the handles opened by the process are closed. 3) Any files that have changed in length and are not closed are not recorded properly in the directory. 4) All memory used by the program is returned to.

14 14 Function 01h Get Keyboard Input Waits for char at STDIN (if necessary), echoes to STDOUT entry AH 01h return AL ASCII character from STDIN (8 bits) note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes INT 23h. 2) For function call 06h, extended ASCII codes require two function calls. 3) The first call returns 00h as an indicator that the next call will be an extended ASCII code. 4) Input and output are re-directable. If redirected, there is no way to detect EOF.

15 15 Function 02h Display Output Outputs char in DL to STDOUT entry AH 02h DL 8 bit data (usually ASCII character) return none note 1) If char is 08 (backspace) the cursor is moved 1 char to the left (nondestructive backspace). 2) If Ctrl-C is detected after input, INT 23h is executed. 3) Input and output are re-directable. If redirected, there is no way to detect disk full.

16 16 Program Structure

17 17 Loading/Running Programs Basic requirements Stack, Data, and Code contents must be in memory SS:SP must point to the top of the Stack DS must point to the default Data Segment CS:IP must point to first instruction in code Simple programs never change SS, DS, or CS Some modular programs use Far calls/jumps for function calls (multiple CS values) Global/local data segments (multiple DS values) Global/local stack segments (multiple SS values)

18 18 Assembly Language Program Files Program file The data, stack, and code part of the program is called the Load Module The information for the operating system is called the program Header Loading and Running OS copies the Load Module into memory OS initializes registers and adjusts stored pointers using Header information OS points CS:IP at the program location

19 19 Program File Types.com file = Load Module Very simple programs Use one only segment CS=DS=SS=ES Do not need a program Header Must start at IP=0100.exe file = Header + Load Module Supports multiple segments and far branches Header contains initial CS:IP, SS:SP, DS Header contains other OS parameters

20 20 Program Segment Prefix (PSP) Created dynamically by when a program is loaded PSP contains program information for use by Location of environment variables Command arguments Other information places PSP into memory just below Load Module PSP length is 256 bytes = 100h bytes For.com program PSP occupies 0000 to 00FF in the single segment Default start IP is in next address (IP=0100)

21 21.COM Programs Program fits into 1 segment: DS=CS=SS=ES Disk file is absolute image of machine code No file header No address adjustments More compact and faster than.exe files does not test.com file for executable code No addresses to adjust loads file after PSP and runs at IP = 0100 Location 0100h must contain an executable instruction Maximum length of.com program is 65,536 bytes - PSP length - mandatory stack word When.COM program runs all segment registers point to PSP

22 22 Structure of a.com Program Store code code Increasing Addresses data and stack code load module load module data and stack code PSP JMP NEAR DS:EA SS:SP start segment = CS:0100 File on Disk File in RAM DS=CS=SS=ES = start segment - 10 Load Module = Code, Program Data, Stack, stored on disk All addresses are relative to the fixed CS=DS=SS=ES Default IP = 0100 Load

23 23 The Program Segment Prefix Offset Size Contents 0000h 2 bytes int 20h (old style program terminate command) 0005h 5 bytes FAR call to MS function dispatcher (int 21h) 000Ah 4 bytes Previous termination handler interrupt vector (int 22h) 000Eh 4 bytes Previous contents of ctrl-c interrupt vector (int 23h) 0012h 4 bytes Previous critical error handler interrupt vector (int 24h) 002Ch 2 bytes Segment address of the program's environment block 0080h 1 byte Parameter length (number of chars entered after filename) and default Disk Transfer Area (DTA) 0081h 127 bytes Parameters command tail

24 24.EXE Program Loading loader checks for program header reads program Header into its work space Start Segment determines a starting address just above the kernel The starting address is a multiple of 10h and is called the Start Segment copies the load module beginning at the Start Segment Program Segment Prefix (PSP) is built just below the Start Segment

25 25 Structure of a.exe Program Store SS SS Increasing Addresses CS stored portion of DS header load module load module CS stored portion of DS PSP data segment SS = start segment + ISS CS = start segment + ICS start segment DS = start segment - 10 File on Disk File in RAM Load Load Module = Program data, stack, code stored on disk ISS = Initial Stack Segment = Offset of SS from start of load module ICS = Initial Code Segment = Offset of CS from start of load module

26 26.EXE Program Relocation 1 SP is set to value from header file IP is set to value from header file Relative addresses converted to absolute addresses: ES and DS registers pointed at PSP Add Start Segment to ISS put result into SS Add Start Segment to ICS put result into CS

27 27.EXE Program Relocation 2 Far Code Module 5678 bytes CS:IP=0234:5678 CS= =2545 Far Code Module Stack Stack Load Module Default Code JMP FAR [1000] Segment Default Data Segment Header File CS= IP= LOAD bytes Start= bytes Default Code Segment Default Data Segment +PSP RAM In.exe file, FAR segment references are relative to the load module. Relocation: After loading the program, Relocation Items (relative FAR references) are made absolute (SEG SEG + start segment)

28 28.EXE Program Header 1 Offset Contents 00h - 01h 4D 5A h This is the Linker's signature to mark the file as a valid.exe file. (The ASCII letters M and Z, for Mark Zbikowski, one of the major designers of at Microsoft) 02h - 03h Length of the image mod h - 05h Size of the file in 512 byte pages (including remainder) including the header. 06h - 07h Number of relocation table items following the header. 08h - 09h Size of the header (including relocation table) in 16 byte increments (paragraphs). 0Ah - 0Bh Minimum number of 16 byte paragraphs required by the program beyond the end of the loaded program (uninitiated data space). 0Ch - 0Dh Maximum number of 16 byte paragraphs required above the end of the loaded program. If the minimum and maximum number of paragraphs are both zero, the program will be loaded as high in memory as possible.

29 29.EXE Program Header 2 0Eh - 0Fh 10h - 11h 12h - 13h 14h - 15h 16h - 17h 18h - 19h 1Ah - 1Bh Displacement in paragraphs of stack segment within load module. This size must be adjusted by relocation. (ISS) Offset that must be in SP register when the load module is given control. (Starting SP) Word Checksum - negative sum of all the words in the file, ignoring overflow. Offset that must be in the IP register when the load module is given control. (Starting IP) Displacement in paragraphs of code segment within load module. This size must be adjusted by relocation. (ICS) Displacement in bytes of the first relocation item in the file. Overlay number (0 for the main resident part of the program)

30 30 Example of Program Header Top of Header (setver.exe) D 5A EF B 00 FF FF D9 02 *MZ......* B 29 4A 0D E B 0D *..{)J...* D D DF 0D A 09 *...U...j.* *...* Offset Value Contents 00h - 01h 4D 5A 4D 5A h 02h - 03h EF 00 Length of the image mod 512 = 00EFh 04h - 05h Size of the file in 512 byte pages (including remainder) including the header = 0018h ( EF = 12, bytes) 06h - 07h Number of relocation table items following the header = h - 09h Size of the header (including relo table) in 16 byte increments (paragraphs) = Ah - 0Bh 8B 00 Minimum number of 16 byte paragraphs required by the program = 008B 0Ch - 0Dh FF FF Maximum number of 16 byte paragraphs required 0Eh - 0Fh D9 02 Displacement in paragraphs of stack segment within load module. ISS= 02D9 10h - 11h Offset that must be in SP register when load module is given control SP = h - 13h 7B 29 Word Checksum - negative sum of all the words in the file, ignoring overflow. 14h - 15h 4A 0D Offset that must be in the IP register when load module is given control IP = 0D4A 16h - 17h Displacement in paragraphs of code segment within load module. ICS = h - 19h 1E 00 Displacement in bytes of the first relocation item in the file = 001E 1Ah - 1Bh Overlay number (0 for the main resident part of the program)

31 31 Example of Program Header Relocation Program loaded at Start Segment = 22CA ICS = 0089 ISS = 02D9 DS = 22CA = 22BA PSP in DS:0000 to DS:00FF CS = 22CA + ICS = 2353 SP = 0800 SS = 22CA + ISS = 25A3 IP = 0D4A Relocation table at 001Eh into header (18h-19h) Item Offset = SEG 10+OFF Location of word in EXE file = load module offset + Item Offset = Item Offset Location of word in RAM = Start Segment + Item Offset = 22CA + Item Offset Relocation Item 1B 0D Seg:Off 0089:0D1B D1B = 15AB Item Offset In File Word Relocation 15AB 17AB CA = 24DC D :08D6 2A36 2C CA = 24DC

32 32 Program Execution Control turned over to program at CS:IP Program finishes Program returns control to : 1. Int 21 Function 4C (Microsoft supported) 2. Int 21 Function 00 (obsolete) 3. Return instruction at end of code (obsolete) 1. CS pointed at PSP 2. Program executes Return instruction 3. IP 0000 popped from top of stack 4. PSP:0000 contains Int 20 (works like Int 21 Function 00)

33 33 Function 0Eh Select Disk Sets the drive specified in DL (if valid) as the default drive entry AL 0Eh DL new default drive number (0=A:,1=B:,2=C:,etc.) return AL total number of logical drives (not necessarily physical)

34 34 Function 19h Get Current Disk Drive Return designation of current default disk drive entry AH 19h return AL current default drive (0=A, 1=B,etc.) note Some other functions use 0 for default, 1=A, 2=B, etc.

35 35 Function 1Ah Set Disk Transfer Area (DTA) Sets DTA address to the address specified in DS:DX entry AH 1Ah DS:DX pointer to buffer return none note 1) The default DTA is 128 bytes at offset 80h in the PSP. uses the DTA for all file I/O. 2) Registers are unchanged. 3) No error codes are returned. 4) Disk transfers cannot wrap around from the end of the segment to the beginning or overflow into another segment.

36 36 Function 2Ah Get Date Returns day of the week, year, month, and date entry AH 2Ah return CX year ( ) DH month (1-12) DL day (1-31) AL weekday 00h Sunday 01h Monday 02h Tuesday 03h Wednesday 04h Thursday 05h Friday 06h Saturday note 1) Date is adjusted automatically if clock rolls over to the next day, and takes leap years and number of days in each month into account. 2) Although cannot set an invalid date, it can read one, such as 1/32/80, etc. 3) will accept CH=0 (midnight) as a valid time, but if a file's time is set to exactly midnight the time will not be displayed by the DIR command.

37 37 Function 2Ch Get Time Get current system time from CLOCK$ driver entry AH 2Ch return CH hours (0-23) CL minutes (0-59) DH seconds (0-59) DL hundredths of a second (0-99) note 1) Time is updated every 5/100 second. 2) The date and time are in binary format

38 38 Function 09h Print String Outputs Characters in the Print String to the STDOUT entry AH 09h DS:DX pointer to the Character String to be displayed return none note 1) The character string in memory must be terminated by a $ (24h) 2) The $ is not displayed. 3) Output to STDOUT is the same as function call 02h.

Ρουτίνες Λειτουργίας (DOS function calls)

Ρουτίνες Λειτουργίας (DOS function calls) Παρακάτω ακολουθεί µία λίστα αυτών των AH κωδικών µε τα ονόµατα της ρουτίνας λειτουργίας (DOS function calls). 00H 01H 02H 03H 04H 05H 06H 07H 08H 09H TERMINATE