Common Software Interrupt Services (MS-DOS)

8088 INTERRUPTS

• Interrupts are a method of accessing operating system services without the requirement of knowing where in memory the specific service needed is located. Access is though a table of addresses of operating system service routines called the "Interrupt Vector Table" kept in a fixed, reserved part of memory. Each address stored in the table is 2 words or 4 bytes long (a 2 byte IP offset address followed by a 2 byte CS segment address - the Little register is stored first).
• The Interrupt Vector Table is always located in low memory at absolute addresses 00000h to 003FFh inclusive. This allows for a maximum of 100h (256d) interrupt service routines. Each service routine is assigned an interrupt number between 00h and FFh.
• Interrupts may be generated by external devices (external hardware interrupts) or by the CPU (processor interrupts).
• Processor interrupts may be caused by processing errors (internal hardware interrupts) such as an attempt to divide by zero or by an INT instruction (software interrupt). For example, "INT 4" tells the CPU to pick up interrupt vector table item 4h and go to that new CS:IP address. Since each interrupt vector address is 4 bytes long, "INT 4" is located at offset 4h*4h=10h or 0000:0010 or real address 00010h. "INT 15" is located at 15h*4h=54h or 0000:0054 or real address 00054h. (Make sure you do your multiplication in hexadecimal, not decimal!)

"MS-DOS" (high-level) Services

"DOS Services" (as opposed to low-level or hardware-level services) are provided through software interrupt number 21(hex.); the specific service being requested is indicated by pre-setting the AH register with a code before performing the "int 21h". The following is a list of a few of the common services available (do not memorize the numbers in this list):

AH = 01h : KEYBOARD INPUT AND DISPLAY

Wait for keyboard character; echo character on display; return ASCII code for character in AL. Terminate program if Ctrl-C or Ctrl-Break is typed.

AH = 07h : KEYBOARD INPUT WITH NO ECHO

Wait for keyboard character; returns ASCII code for character in AL. Does not check for Ctrl-C or Ctrl-Break.

AH = 0Bh : KEYBOARD STATUS

If a character is available from the keyboard, returns FFh in AL; otherwise returns 00h in AL.

AH = 02h : DISPLAY CHARACTER

Display character with ASCII code pre-stored in DL.

AH = 09h : DISPLAY STRING

Display string of characters found in memory starting at the address given by [DS:DX] and ending with a "$" (an ASCII$ string); the "$" character is not displayed.

AH = 4Ch : TERMINATE PROGRAM WITH ERRORLEVEL

Terminate program execution and return an "errorlevel" code equal to the value in the AL register; a value of 00h normally is used to indicate a normal (non-abort) program termination. For .COM-style programs (only) an alternate method of program termination is with a different interrupt, "int 20h"; note that this method only works properly with .COM programs and does not allow for a return code value.

(Do not memorize the numbers in the above list.)

Note the "keyboard" and "display" services described above really apply to "standard input" and "standard output" and thus allow for "piping" to/from other programs or "redirection" to/from files.