ACA

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UNIT 1
Define computer Architecture. Illustrate the seven dimensions of an ISA?(10 marks ) ( June 2012) (Dec 2012)(June 2011 )(June 2013)(Dec 2013)(Jan 2014)
The computer designer has to ascertain the attributes that are important for a new computer and design the system to maximize the performance while staying within cost, power and availability constraints . The task has few important aspects such as Instruction set design, Functional organization, Logic design and implementation .
Instruction set Architecture (ISA)
ISA refers to the actual programmer visible Instruction set. The ISA serves as boundary between the 
software and hardware . The seven dimensions of the ISA are : 

i)class of ISA : Nearly all ISAs today are classified as General -Purpose - Register architectures . 
The operands are either Registers or Memory locations . 
The two popular versions of this class are: Register-Memory ISA'S : ISA of 80x86, can access memory as part of many instructions .
Load - store I SA Eg. ISA of MIPs, can access memory only with Load or store instructions .


ii) Memory addres sing : Byte addressing scheme is most widely used in all desktop and server computers . Both 80x86 and MIPs use byte addressing . Incase of MIPs the object must be aligned . An access to an object of s byte at byte address A is aligned if A mod s =0 . 80x86 does not require alignment . Accesses are faster if operands are aligned.
iii) Addressing modes :specify the address of a Mobject apart from register and constant operands .
MIPs Addressing modes :
•Register mode addressing
• Immediate mode addressing
•Displacement mode addressing
80x86 in addition to the above addressing modes supports the additional modes of addressing:
i. Register Indirect
ii. Indexed
iii,Based with scaled index
iv)Types and sizes of operands:
MIP s and x 86 support :
• 8 bit (ASCII character) , 16 bit• 
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32 b it (Integer/word ) 
• 64 b it (long integer/ Double word)
• 32 b it (IEEE- 754 f loating point)
• 64 b it (Double precision floating point)
• 80 x 86 also supports 80 bit f loating point operand. (extended double Precision


v)operation s : The general category of operations are :
o-Data Transfer
o-Arithmetic operations
o-Logic operations
o-control operations
o-MIP s IsA : simple & easy to implement
ox86 IsA : richer & larger set of operations


vi) control flow instructions :All ISA'S support :
conditional & unconditional Branches
Procedure calls & Returns MIPs 80x86
• conditional Branches tests content of Register condition code bits • Procedure call JAL CALLF
• Return Address in a R stack in M


vii) Encoding an Is A
Fixed Length ISA variable Length ISA
MIP' S 32 B it long 80x 86 ( 1 - 18 bytes)
s implif ie s decoding Take s les s space
Number of Registers and number of Addressing modes hav e significant impact on the length of instruction as the register field and addressing mode field can appear many times in a s ingle instruction.


2.what is dependability? Explain the two measures of Dependability? (06 marks) (May/June 2012 ) (Dec 2012 ) ?

The Infrastructure providers offer service Level Agreement (SLA) or service Level objectives (SLO) to guarantee that their networking or power services would be dependable .
• Systems alternate between 2 states of service with respect to an
SLA:
. service accomplishment , where the service is delivered as specified in SLA . service interruption , where the delivered service is different from the SLA • Failure = transition from state 1 to state 2
• Restoration = transition from state 2 to state 1

The two main measures of Dependability are Module Reliability and Module Availability. Module reliability is a measure of continuous service accomplishment (or (unicode character)
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