Chapter 6: Program Control Instructions
6–4 INTRO TO INTERRUPTS
• An interrupt is a hardware-generated CALL
– externally derived from a hardware signal
• Or a software-generated CALL
– internally derived from the execution of an
instruction or by some other internal event
– at times an internal interrupt is called an exception
• Either type interrupts the program by calling
an interrupt service procedure (ISP) or
interrupt handler.
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١
Interrupt Vectors
• A 4-byte number stored in the first 1024 bytes
of memory (00000H–003FFH) in real mode.
– in protected mode, the vector table is replaced by
an interrupt descriptor table that uses 8-byte
descriptors to describe each of the interrupts
• 256 different interrupt vectors.
– each vector contains the address of an interrupt
service procedure
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• Each vector contains a value for IP and CS
that forms the address of the interrupt service
procedure.
– the first 2 bytes contain IP; the last 2 bytes CS
• Some reserved vectors are for errors that
occur during the execution of software
– such as the divide error interrupt
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٢
Interrupt Vectoring Process
Interrupt Handler
Calling program
mov... F000:F065 sti 3
int 10h F066 cld
add... F067 push es
1 2 F068 .
. .
. IRET
3069 F000:F065 F000:AB62
(entry for INT 10)
Interrupt Vector Table
4
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Interrupt Instructions
• Three different interrupt instructions available:
– INT, INTO, and INT 3
• In real mode, each fetches a vector from the
vector table, and then calls the procedure
stored at the location addressed by the vector.
• In protected mode, each fetches an interrupt
descriptor from the interrupt descriptor table.
• Similar to a far CALL instruction because it
places the return address (IP/EIP and CS)
on the stack.
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٣
INTs
• 256 different software interrupt instructions
(INTs) available to the programmer.
– each INT instruction has a numeric operand
whose range is 0 to 255 (00H–FFH)
• For example, INT 100 uses interrupt vector
100.
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• Address of the interrupt vector is determined
by multiplying the interrupt type number by 4.
– INT 10H instruction calls the interrupt service
procedure whose address is stored beginning at
memory location 40H (10H 4) in the mode
• In protected mode, the interrupt descriptor is
located by multiplying the type number by 8
– because each descriptor is 8 bytes long
• Each INT instruction is 2 bytes long.
– the first byte contains the opcode
– the second byte contains the vector type number
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٤
• When a software interrupt executes, it:
– pushes the flags onto the stack
– clears the T and I flag bits
– pushes CS onto the stack
– fetches the new value for CS from the
interrupt vector
– pushes IP/EIP onto the stack
– fetches the new value for IP/EIP from
the vector
– jumps to the new location addressed by
CS and IP/EIP
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• INT performs as CALL
– not only pushes CS & IP onto the stack, also
pushes the flags onto the stack
• Software interrupts are most commonly used
to call system procedures because the
address of the function need not be known.
• The interrupts often control printers, video
displays, and disk drives.
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Barry B. Brey
٥
Common Interrupts
Software interrupts will call interrupt service
routines (ISRs) either in BIOS or DOS
• INT 10h Video Services
• INT 16h Keyboard Services
• INT 17h Printer Services
• INT 1Ah Time of Day
• INT 1Ch User Timer Interrupt
• INT 21h MS-DOS Services
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Function 4Ch of INT 21h
• Terminate process:
– Ends the current process (program), returns an
optional 8-bit return code to the calling process.
– A return code of 0 usually indicates successful
completion.
mov ah,4Ch ; terminate process
mov al,0 ; return code
int 21h
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٦
Function 09h of INT 21h
• INT 21h: invoke MS-DOS services
– Function code in AH, e.g. 09H = write string
– The string must be terminated by a '$' character.
– DS must point to the string's segment, and DX
must contain the string's offset.
mov ah,9
mov dx,OFFSET string
int 21h
string DB ‘Hello, World!$’
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Selected I/O Functions
• Output functions:
– 02h, 06h - Write character to standard output
– 05h - Write character to default printer
– 09h - Write string to standard output
– 40h - Write string to file or device
• Input functions:
– 01h, 06h - Read character from standard input
– 0Ah - Read array of buffered characters from standard
input
– 0Bh - Get status of the standard input buffer
– 3Fh - Read from file or device
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٧
IRET/IRETD
• Used only with software or hardware interrupt
service procedures.
• IRET is used in real mode and IRETD in the
protected mode.
• IRET instruction will
– pop stack data back into the IP
– pop stack data back into CS
– pop stack data back into the flag register
• Accomplishes the same tasks as the POPF
followed by a far RET instruction.
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INT 3
• A special software interrupt designed to
function as a breakpoint.
– a 1-byte instruction, while others are 2-byte
• Common to insert an INT 3 in software to
interrupt or break the flow of the software.
– function is called a breakpoint
– breakpoints help to debug faulty software
• A breakpoint occurs for any software interrupt,
but because INT 3 is 1 byte long, it is easier to
use for this function.
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Barry B. Brey
٨
INTO
• Interrupt on overflow (INTO) is a conditional
software interrupt that tests overflow flag (O).
– If O = 0, INTO performs no operation
– if O = 1 and an INTO executes, an interrupt
occurs via vector type number 4
• The INTO instruction appears in software that
adds or subtracts signed binary numbers.
• JO or INTO instructions detect the overflow.
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Interrupt Control
• Two instructions control the INTR pin.
• The set interrupt flag instruction (STI) places
1 in the I flag bit.
– which enables the INTR pin
• The clear interrupt flag instruction (CLI)
places a 0 into the I flag bit.
– which disables the INTR pin
• The STI instruction enables INTR and the CLI
instruction disables INTR.
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٩
6–5 MACHINE CONTROL AND
MISCELLANEOUS INSTRUCTIONS
• These instructions provide control and perform
various other functions.
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HLT
• Stops the execution of software.
• DOS and Windows both use interrupts
extensively.
– so HLT will not halt the computer when
operated under these operating systems
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Barry B. Brey
١٠
NOP
• In early years, before software development
tools were available, a NOP, which performs
absolutely no operation, was often used to pad
software with space for future machine
language instructions.
• When the microprocessor encounters a NOP,
it takes a short time to execute.
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• If you are developing machine language
programs, which are extremely rare, it is
recommended that you place 10 or so NOPS
in your program at 50-byte intervals.
– in case you need to add instructions at some
future point
• A NOP may also find application in time
delays to waste time.
• A NOP used for timing is not very accurate.
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Pentium Pro Processor, Pentium II, Pentium, 4, and Core2 with 64-bit Extensions Copyright ©2009 by Pearson Education, Inc.
Architecture, Programming, and Interfacing, Eighth Edition Upper Saddle River, New Jersey 07458 • All rights reserved.
Barry B. Brey
١١
