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". Each address is 2 words or 4 bytes long (a 2 byte offset address followed by a 2 byte segment address).
• The Interrupt Vector Table is always located at absolute addresses 00000 to 003FFh inclusive. This allows for a maximum of 100h (256d) interrupt service routines. Each service routine is assigned an interrupt type 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).

"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:

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.