Flag manipulation and processor control instructions , Assembly Language

Flag Manipulation and Processor Control Instructions

These instructions control the functioning of available hardware inside the processor chip. These are categorized into these types; (a) machine control instructions and (b) flag manipulation instructions. The flag manipulation instructions are directly modify some of the flags of 8086. The machine control instructions can control the usage and execution of the bus. The flag manipulation instructions and their functions are following:

CLC -   Clear carry flag

CMC -   Complement carry flag

STC -   Set carry flag

CLD -   Clear direction flag STD:-Set direction flag CLI - Clear interrupt flag STI - Set interrupt flag

These instructions modify directly the direction (DF), carry (CF) and interrupt (IF) flags. The DF and IF flag, which can be modified by using the flag manipulation instructions, further control the processor operation; like the auto increment or auto decrement and the interrupt responses modes. Thus the respective instructions may also be called as processor or machine control instructions. The other flags may be modified by using POPF  and SAHF instructions, which are termed as the data transfer instructions. No direct instructions are available for modifying the status flags except the carry flag.

The machine control instructions supported by the 8086 and 8088 are listed along with their functions below. These machine control instructions do not need any operand.

 

WAIT   -   Wait for Test input pin to go low

HLT      -   Halt the processor

NOP     -   No operation

ESC      -   Escape to external device like NDP (numeric co-processor)

LOCK   -   Bus lock instruction prefix.

After execution of the HLT instruction, the processor goes to the halt state, as explained in first Chapter. The two ways to pull it out of the halt state are to reset processor or interrupt it. When instruction NOP is executed, the processor does not execute any operation until the 4 clock cycles, except incrementing the IP by one. Then it continues with further execution after the 4 clock cycles. When executed ESC instruction, frees the bus for an external master like a peripheral or coprocessor devices.  The LOCK prefix may seem with another instruction.  When it is executed, the bus access is not endorsed for another master until the lock prefixed   instruction is executed fully. This instruction is utilized in the case of programming for multiprocessor systems. The instruction WAIT when executed holds the operation of processor with the current status until the logic level on the TEST pin set low. The processor goes on inserting WAIT states in the instruction cycle, until the TEST pin set low. Once the TEST pin set low, it continues with further execution.

 

Posted Date: 10/12/2012 5:19:36 AM | Location : United States






Your posts are moderated
Related Questions
How do i convert a asci number to numerals?

DMA Hardware (8237 DMAC) :   1)Processor contain HOLD/HOLD Acknowledge lines to interact with 8237 o   DMAC can achieve control of ISA bus by asserting HOLD o   P

This is a short program to practice assembly language loops and if/else statements. You will use various jump commands and the cmp instruction. The program will generate a random

I am assigned to implement dijkstra algorithm in assembly language. I am not a novice in assembly. I need help implementing it.Kindly if anyone then please.

CMP: Compare: - This instruction compares the source operand, which can be a register or memory location an immediate data with a destination operand that might be a register or a

AND: Logical AND: This instruction bit by bit ANDs the source operand that might be an immediate, or a memory location or register to the destination operand that might be a memor

Using the AddSub program from Ch3 under c:\Masm615\examples as a reference, write a program that subtracts three 16-bit integers using only registers. Insert a call DumpRegs statem

I need a division subroutine. Asks for two inputs, then displays the inputs and shows the answer with a remainder. Mine isnt displaying the inputs correctly.


hi!im looking for someone who expert in an assembly language and help me write the programmed!Thank you