Gerhard Roehrl

  • The purpose of this manual is two-fold. First, we will provide a brief description of how the 8086/88 microprocessors handle interrupts. Second, we will give an overview of interrupt processing. Finally, we will describe how to use the system service routines in PC/MS-DOS and how to write both Assembly Language and PASCAL programs that make use of PC/MS-DOS system services.

    1.1 Interrupt Sources

    The 8086/88 microprocessors allow normal program execution to be interrupted by external signals or by special instructions embedded in the program code. When the microprocessor is interrupted, it stops executing the current program and calls a procedure which services the interrupt. At the end of the interrupt service routine, the code execution sequence is returned to the original, interrupted program.

    An interrupt can be generated by one of three sources. First, an interrupt can be generated as a result of a processor state violation, called an exception. An example would be a divide-by-zero interrupt produced when the DIV instruction is interpreted to have a zero divisor. Program execution is automatically interrupted and control transferred to an interrupt handler. Conditional interrupts such as this are referred to as internal interrupts.

    An interrupt can also be generated by an external device requesting service. This happens when a device signals its request on either the non-maskable interrupt (NMI) or on the INTR interrupt input lines of the processor. The NMI interrupt is generally used to signal the occurrence of a catastrophic event, such as the immanent loss of power. The INTR interrupt is used by all other devices. An interrupt caused by a signal applied to either the NMI or INTR input is referred to as a hardware interrupt.

    Since there is only one INTR input, and multiple devices may have an interrupt capability, an Intel 8259A Programmable Interrupt Controller (PIC) can be used to manage multiple interrupt requests. The PIC receives requests from peripheral equipment, decides which request has the highest priority and issues an interrupt request to the CPU.

    Finally, interrupts may be generated as a result of executing the INT instruction. This is referred to as a software interrupt.

    1.2 Interrupt Vectoring

    Interrupt service routines are called in a manner similar to FAR procedures. Two 16 bit data words are used to specify the location of the interrupt service routine. one word is used to load the CS register and points to the base address of the code segment containing the service routine. The second word is used to load the IP with the offset value for the desired routine within the specified code segment.

    The base and offset words for all interrupt types are grouped together in an interrupt vector table. Figure 1 shows the vector table for the 86/88 processors. Note that interrupt vectors are assigned a location based on the type of the interrupt. Although some of the 256 possible interrupt vectors are reserved by Intel, most are available to the programmer.

    Finally, in the IBM PC (and of course, like-wise in the ATT 6300) some of the interrupt TYPES are reserved for PC/MS-DOS system service routines. Figure 2 shows which interrupt types are reserved for this case.

    Interrup                                      Interrup                                       
    t                                             t                                              
    Dec      Hex   Address  Use                   Dec      Hex   Address  Use                    
    0        0     0000     Generated by CPU      26       1A    0068     Invokes time and date  
                            when division by                              services in BIOS       
                            zero is attempted                                                    
    1        1     0004     Used to single-step   27       1B    006C     Interrupt generated    
                            through programs (as                          on keyboard break      
                            with DEBUG)                                   under BIOS; a routine  
                                                                          is invoked if we       
                                                                          create it              
    2        2     0008     Non-maskable          28       1C    0070     Interrupt generated    
                            interrupt; in PCir,                           at each clock tick; a  
                            NMI has some special                          routine is invoked if  
                            uses                                          we create it           
    3        3     000C     Used to set           29       1D    0074     Points to table of     
                            break-points in                               video control          
                            programs (as with                             parameters             
    4        4     0010     Generated when        30       1E    0078     Points to disk base    
                            arithmetic result                             table                  
    5        5     0014     Invokes print-screen  31       1F    007C     Points to high video   
                            service routine in                            graphics characters    
    8        8     0020     Generated by          32       20    0080     Invokes                
                            hardware clock tick                           program-terminate      
                                                                          service in DOS         
    9        9     0024     In most models,       33       21    0084     Invokes all            
                            generated by                                  function-call          
                            keyboard action;                              services in DOS        
                            simulated on PCjr                                                    
                            for model                                                            
    13       D     0034     Generated during CRT  34       22    0088     If we create it, an    
                            vertical retrace for                          interrupt routine is   
                            video control                                 invoked at program     
                                                                          end under DOS          
    14       E     0038     Signals diskette      35       23    008C     If we create it, an    
                            attention (e.g. to                            interrupt routine is   
                            signal completion)                            invoked on keyboard    
                                                                          break under DOS        
    15       F     003C     Used in printer       36       24    0090     If we create it, an    
                            control                                       interrupt routine is   
                                                                          invoked at critical    
                                                                          error under DOS        
    16       10    0040     Invokes video         37       25    0094     Invokes absolute       
                            display services in                           diskette read service  
                            BIOS                                          in DOS                 
    17       11    0044     Invokes               38       26    0098     Invokes absolute       
                            equipment-list                                diskette write         
                            service in BIOS                               service in DOS         
    18       12    0048     Invokes memory-size   39       27    009C     Ends program, but      
                            service in BIOS                               keeps it in memory-    
                                                                          under DOS              
    19       13    004C     Invokes diskette      68       44    0110     Points to low video    
                            services in BIOS                              graphics characters;   
                                                                          only on PCjr           
    20       14    0050     Invokes               72       48    0120     Invokes program to     
                            communications                                translate PCjr         
                            services in BIOS                              keyboard into PC       
    21       15    0054     Invokes cassette      73       49    0124     Points to translation  
                            tape services in                              table for              
                            BIOS                                          keyboard-supplement    
    22       16    0058     Invokes standard                                                     
                            keyboard services in                                                 
    23       17    005C     Invokes printer                                                      
                            services in BIOS                                                     
    24       18    0060     Activates ROM-BASIC                                                  
                            language, or                                                         
                            override for it                                                      
    25       19    0064     Invokes boot-strap                                                   
                            start-up routine in                                                  

    Figure 2 The interrupt TYPES reserved for use by the IBM family of personal computers. A comparison of the reserved locations in Figures 1 and 2 show that some of the locations are actually reserved by the processor manufacturer for specified exceptions (e.g., TYPES 0-4).


    Figure 3 presents, in flowchart form, how the 86/88 processors handle internal, nonmaskable (NMI), single step and INTR interrupts.

    If an INTR interrupt occurs, the CPU checks to see if this kind of interrupt is enabled (through the use of the STI instruction) and if so, acknowledges the interrupt request by generating an interrupt acknowledge (IAK) bus cycle. As a result of the IAK bus cycle, the uP reads an 8 bit TYPE number provided by the interrupting device. This TYPE number is used as an index into the interrupt vector table to get the address of the interrupt handler.

    If Trap Flag (TF) is set, the single step operation is activated. After every instruction, a Type I interrupt is generated and a single step interrupt handler is called. If several interrupts occur simultaneously, the single step function is temporarily disabled during the execution of the INTR vectored interrupt service routine.

    2.1 INT Instruction

    Software interrupts produced by the INT assembler instruction have many uses. For example, one use would be for the testing of the various interrupt service routines. You could use an INT 2 instruction to start the execution of an NMI interrupt service procedure. This would allow you to test the NMI procedure without needing to apply an external signal to the processors NMI input line.

    Software interrupts can also be used to call commonly used procedures from many different programs. The Basic Input/Output System (BIOS) procedures of an IBM computer or compatible are a good example of this use of the INT instruction. The specific functions and use of these procedures will be covered in detail in Section 3, below.

    Figure 3 The interrupt recognition and processing sequence for the 8086/88 family of processors is shown in flowchart form.

    In general, INT activates the interrupt procedure specified by the instruction's TYPE operand. To return from an interrupt service routine, the IRET instruction is used. IRET is used to exit any interrupt procedure, whether activated by hardware or software. The specific action of INT and IRET can be found in any reference on the 8086/88 instruction set.

    2.2 Loading and Examining IR Pointers

    To be able to vector to your own interrupt service routines, you have to load the interrupt pointer table with the complete two-word start address of your interrupt hander corresponding to the desired interrupt TYPE number. The following example shows how this is done using interrupt type 36. Good programming practices dictate that you should first store the original contents of the part of the pointer table you want to change. The following code segment does this operation.

    xor ax, ax ; first clear the AX register

    mov es, ax ; load ES with segment address

    ; of interrupt pointer table

    ; (use 0000h for this example)

    mov bx, 36 ; load BX with interrupt type

    mov ax, word ptr es:[bx] ; load AX with the offset

    ; for interrupt type 36

    push ax ; save the old interrupt pointer

    ; table content

    mov ax, word ptr es:[bx+2] ; Load AX with the segment

    ; address of interrupt type 36

    push ax ; store the old interrupt pointer

    ; table content

    Now, the interrupt table can be loaded with the new vector locations for the handler you have written.

    xor ax, ax ; first clear AX

    mov es, ax ; load ES with the segment address

    ; for the interrupt pointer table

    mov bx, 36 ; load BX with interrupt type

    cli ; disable interrupts during the

    ; changes to the interrupt table

    mov word ptr es:[bx], BBBB ; load the lower interrupt pointer

    ; table word with the base address

    ; (BBBBH) of the user's interrupt

    ; service routine

    mov word ptr es:[bx+2],SSSS ; load the higher interrupt pointer

    ; table word with the segment address

    ; (SSSSH) of the user's interrupt

    ; service routine

    sti ; re-enable interrupts

    3.0 THE ROM BIOS

    3.1 What is the BIOS?

    The ROM BIOS (Basic Input Output System) is part of the ROM based control -system of an IBM PC or compatible that both defines the architecture of the computer to the software, and provides the fundamental I/O services that are needed for the operation of the computer.

    The BIOS is actually a collection of procedures. Each procedure performs a specific function such as reading a character from the keyboard, writing characters to the screen, or reading information from disk. The ROM BIOS in the AT&T PC 6300 is located starting at physical address FC000 (F000:C000).

    3.2 Using BIOS Procedures

    System I/O procedures are called with the INT instruction (Section 2). The advantage to calling procedures using this instruction is that the programmer need not know the absolute address of the procedure being called or how to link the procedures into the calling program. All you have to know is the interrupt TYPE for the procedure and the format for the parameter(s) that have to be passed to the procedure.

    There are twelve BIOS interrupts in all, falling into five groups (Figure 4). For example with INT 10h you can access the video display services. This interrupt includes 20 subroutines. Obviously, one of the INT 10h parameters is a data value indicating which one of the twenty subroutines is required. In this case, the AH Register is loaded with the number of the subroutine. In addition, the AL, BX, CX and DX registers are used to provide the parameters for this subroutines.

    Dec        Hex              Use                                                   
    Peripheral Devices Services                                                     
        16            10        Video-display services                                
        19            13        Diskette services                                     
        20            14        Communications services                               
        21            15        Cassette-tape services                                
        22            16        Standard keyboard services                            
        23            17        Printer services                                      
    Equipment Status Services                                                       
        17            11        Equipment-list service                                
        18            12        Memory-size service                                   
    Time/Date Service                                                               
        26            1A        Time and date services                                
    Print-Screen Service                                                            
        5             5         Print-screen service                                  
    Special Services                                                                
        24            18        Activate ROM-BASIC language                           
        25            19        Activate bootstrap start-up routine                   

    Figure 4 The twelve BIOS service routines supported by the IBM PC (and compatibles).

    As a simple example of the use of the BIOS routines, let us assume that we want to set the AT&T PC 6300 monitor to the super high graphic resolution mode 64Ox4OO pixel. Normally the is a tricky programming job if you program the video display controller hardware directly. However, when using the BIOS routines, the control change is extremely simple.

    mov ah,00h ; load AH with service

    ; number 0 = "Set video model"

    mov al,48h ; load AL with mode number

    ; 48 = "Super high resolution model"

    int 10h ; call the BIOS procedure to set

    ; the desired video mode

    As a second example we will write some pixels to the screen and create graphics. The code shown below will turn ON one pixel at screen position (0,639):

    mov ah,OCh ; load number OCh = "write

    ; pixel dot"

    mov al,O1h ; set code for pixel color

    mov dx,0000h ; load DX with row number = 0

    mov cx,027Fh ; Load CX with column number = 639d

    int 10h ; call the BIOS procedure to set

    ; one pixel on the screen

    Finally, examine the organization and operation of a program that will send characters to the printer. Note that the BIOS interrupt 17h is used.

    ; this program initializes the

    ; printer port and sends a character

    ; string to the printer

    code segment ; defines that this is a code segment

    org 100h ; starting at offset 100h

    assume cs:code, ds:code ; and assuming the registers CS and

    ; DS are loaded with the segment

    ; address for this routine

    ; for more information, see the

    ; TURBO-ASSEMBLER manual!

    enter proc far; ; this procedure (ENTER) will simply

    call print ; call the PRINT routine written below

    ret ; info: see TURBO-ASSEMBLER manual

    enter endp

    message db 'Hallo printer, how are you?'

    db 0dh, 0ah ; a carriage return & line feed is used

    ; to terminate string messages

    print proc near

    mov ah, 01h ; Service number 01 = "Initialize

    ; printer port"

    mov dx, 00h ; Use printer port 0

    int 17h ; Call procedure to initialize

    ; printer

    mov si, offset message ; Load SI with offset of string

    mov cs,29 ; Load CX with length of string

    again: mov ah, 00h ; service number 01 = "Print

    ; character"

    move al, byte ptr [si] ; Load character to be sent in AL

    int 17h ; Call procedure to print character

    cmp ah, 01h ; If character not printed then AH=1

    jnz next

    jmp exit

    next: inc si ; Address of next character

    loop again ; Send next character

    exit: ret

    print endp

    code ends


    Note: If you use BIOS Interrupts, make sure, that you save all registers which could be affected by the BIOS routine. Reference [4] provides more details on the operation of the various BIOS routines.


    4.1 Comparing DOS and BIOS Services

    When you turn on your PC there are several jobs to do. One is to load the operating system from the system disk. If you use MS-DOS (MicroSoft - Disk Operating System), three system files are loaded; IBMBIO.COM, COMMAND.COM and IBMDOS.COM.

    The file IBMDOS.COM contains DOS service routines. The DOS services, like the BIOS services, can be called by programs through a set of interrupts whose vectors are placed in the interrupt vector table (Section 1.2). The ROM-BIOS routines can be thought of as the lowest-level system software available, performing the most fundamental and primitive input and output operations. The DOS service routines provide more sophisticated and efficient control over the I/O operations than the BIOS routines do, particularly for disk file operations.

    4.2 Using DOS interrupts

    There are nine DOS interrupt services and they are listed in Figure 5. Five of them, interrupts 20h, 25h through 27h and 2fh are "true" DOS interrupt services, each one having a specifically-defined task associated with it.

               Dec                         Hex                    Description         
                32                         20              Program terminate: come    
                                                           to normal ending           
                33                         21              Function-call umbrella     
                34                         22              Terminate address          
                35                         23              Break address              
                36                         24              Critical error-handler     
                37                         25              Absolute disk read         
                38                         26              Absolute disk write        
                39                         27              Terminate-but-stay-residen 
                47                         2F              Print spool control        
                                                           (DOS-3 versions only)      

    Figure 5 The Nine DOS Interrupts.

    In Section 4.4 we will examine the DOS service routine called by INT 21h. This routine provides under one "umbrella" a set of universal functions we can use in our programs.

    4.3 DOS Interrupts Except INT 21h

    INT 20h "Program Terminate"

    This interrupt terminates the current process and returns control back to the parent process. For example, if you run a com.file program, INT 20 terminates your program and returns to DOS.

    INT 22h, INT 23h, INT 24h

    This three interrupts are used to hold segmented addresses. our programs set these addresses to point to special routines. Then, when the appropriate circumstances arise, DOS invokes the routines located at these addresses through these three address interrupts. This interrupts are used for advanced programming. More information about these interrupts are provided in references [1] and [4].

    INT 25h, INT 26h

    These two interrupts are used to read and write specific disk sectors. They are the only DOS services that ignore the logical structure of a disk and work with individual sectors.

    INT 27h "Terminate But Stay Resident"

    The Terminate But Stay Resident call is used to establish a section of code as resident in the system after its termination. This system call is often used to install device drivers in memory.

    4.4 The Universal Functions of DOS INT 23Lh

    All of the DOS function calls are invoked by INT 21h. Individual functions are selected in the same way as BIOS functions, placing the function number in the AH-Register.

    To see how easy the use of DOS functions are, compare the following sample programs with the programs in Section 2.2.

    Function "Get Interrupt Vector"

    mov ah,35h ; function number in AH-Register

    mov al,36 ; interrupt type in AL-Register

    int 21h ; get interrupt vector

    push bx ; save offset address

    push es ; save segment address

    Function "Set Interrupt Vector

    mov dx,xxxx ; offset address in DX-Register

    mov ds,yyyy ; segment address in DS-Register

    mov ah,25h ; function number in AH-Register

    mov al,36 ; interrupt type in AL-Register

    int 21h ; set interrupt vector

    The program "Stopwatch" shows you the use of some other INT 21h functions. More information about these functions is provided in reference [1] and [4].

                    Description                     PCjr        PC/XT          AT      
    DMA controller (8237A-5)                        n/a        000-00F      000-01F    
    Interrupt controller (8259A)                  020-027      020-021      020-03F    
    Timer (8253-5; 8254.2 in AT)                  040-047      040-043      040-05F    
    PPI (8255A-5; 8255-5 in PCir)                 060-067      060-063        n/a      
    Keyboard (8042)                                 n/a          n/a        060-06F    
    DMA page register (74LS612)                     n/a        080-083      080-09F    
    NMI  (non-maskable interrupt) mask            0A0-0A7         0A        070-07F    
    Interrupt controller 2 (8259A)                  n/a          n/a        0A0-0BF    
    Sound generator (SX76496N)                    0C0-0C7        n/a          n/a      
    DMA controller 2 (8237A-5)                      n/a          n/a        0C0-0DF    
    Clear/reset math coprocessor                    n/a          n/a        0F0-0F1    
    Math coprocessor                                n/a          n/a        0F8-0FF    
    Joystick (game controller)                    200-207      200-20F      200-207    
    Expansion unit                                  n/a        210-217        n/a      
    Parallel printer (secondary)                    n/a          n/a        278-27F    
    Serial port (primary)                         2F8-2FF      3F8-33FF     3F8-3FF    
    Serial port (secondary)                         n/a        2F8-2FF      2F8-2FF    
    Prototype card                                  n/a        300-31F      300-31F    
    Fixed disk                                      n/a        320-32F      1F0-1F8    
    Parallel printer (primary)                      n/a        378-37F      378-37F    
    SDLC (secondary bisynchronous                   n/a        380-38F      380-38F    
    communications in AT only)                                                         
    Bisynchronous communications (primary)          n/a          n/a        3A0-3AF    
    Monochrome adapter/printer                      n/a        3B0-3BF      3B0-3BF    
    Color/graphics adapter                          n/a        3D0-3DF      3D0-3DF    
    Diskette controller                           0F0-0FF      3F0-3F7      3F0-3F7