Read after write and write after write - data hazards, Computer Engineering

RAW and WAW - Data hazards:

RAW (read after write) - j tries to read a source before i writes it, hence j wrongly gets the old value .This is the most usual type of hazard and the kind that we use forwarding to solve it.

WAW (write after write) - j tries to write an operand before i. The writes end up being performed in the incorrect order, leaving the value written by  i rather than the value written by  j in the destination. This hazard is there only in pipelines that write in more than 1 pipe stage or permit an instruction to proceed even when a preceding instruction is stalled. DLX integer pipeline writes a register just in WB and ignores this class of hazards.

WAW hazards would be possible if we perform the following 2 changes to the DLX pipeline:1)move write back for an ALU operation into the MEM stage, while the data value is available by then. Assume that the data memory access took 2 pipe stages.

Here is a sequence of 2 instructions which showing the execution in this revised pipeline, importance the pipe stage that writes the consequence:

1753_Read after write and write after write - data hazards.png

 Unless this type of hazard is ignored, execution of this sequence on this revised pipeline will leave the result of the first write (the LW) in R1, before the result of the instruction ADD.

Let writes in different pipe stages introduces other problems, since 2 instructions may try to write at the similar clock cycle. DLX FP pipeline, which has both writes in different pipeline lengths and different stages and will deal with both write conflicts and WAW hazards in detail.

Posted Date: 10/13/2012 4:07:06 AM | Location : United States







Related Discussions:- Read after write and write after write - data hazards, Assignment Help, Ask Question on Read after write and write after write - data hazards, Get Answer, Expert's Help, Read after write and write after write - data hazards Discussions

Write discussion on Read after write and write after write - data hazards
Your posts are moderated
Related Questions

i need help with an octave program

COMPUTER FUNDAMENTALS 1. Elaborate the various steps in performing a Mail Merge. Perform one mail merge operation for sending invitation for a conference which is being conduc

What is the difference between field-group header and other field groups? The header field group is a particular field group for the sort criteria.  The system automatically p

Illustrate the Encryption and Decryption methods One of the best method of ensuring security is to change the form of communication. Let the messages be encoded in such a patte

Explain the term- macro? A term macro is a set of instructions, which can be executed repeatedly. It is useful for automating certain routine tasks like printing reports etc. T

Intentional hacking helps in maintaining better security Several companies employ professionals as security specialists whose basic job is to detect and cover loopholes in t

Write a decision table that specifies the rules for the game of checkers

Q. Explain about Algebraic Simplification? We have already determined algebraic simplification of logic circuit. An algebraic expression can exist in SOP or POS forms.  Let's i

Multiple Instruction and Single Data stream (MISD): In this type of organization multiple processing elements are ordered under the control of multiple control units. Every contro