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







Related Discussions:- Flag manipulation and processor control instructions , Assignment Help, Ask Question on Flag manipulation and processor control instructions , Get Answer, Expert's Help, Flag manipulation and processor control instructions Discussions

Write discussion on Flag manipulation and processor control instructions
Your posts are moderated
Related Questions
IRET : Return from ISR:- When an interrupt service routine is called, before transferring control to it, the IP, CS  register and flag registers are stored in the stack to ment

Any small project which can implement on any software. No need any external hardware approach.

SBB: Subtract  with Borrow  :- The subtract with borrow instruction subtracts the source operand and the borrow flag (CF) which might reflect the result of the past calculations,

ASSUME: Assume Logical Segment Name:- The ASSUME directive which is used to inform the assembler, the specified names of the logical segments to be consider for different segme

How is bob different from johnny in the outsiders

code to add two matrices

MLIL: Unsigned Multiplication Byte or Word: This instruction multiplies an unsigned byte or word by the contents of the AL. The unsigned byte or word can be in any one of the gene

Addressing mode of 8086 : Addressing mode specify a way of locating operands or data. Depending on the data types used the memory  addressing  modes and in the instruction  ,

Write an assembly language program that will display (print) a list of the Decades 2010, 2020, 2030... 2100 to the screen using a while loop.

8279 Keyword /Display Controller : Figure shows the structure of 8279 and its interface to the bus. Addressing is according to the table given below. CS        RD