Doubt in CSO

Question

Q1) How processor knows the instruction is one word or more?

Q2) Is it possible that opcode in instruction is greater than word size?

Q3) What is the format of PSW and how many PSW registers are there?

Comments

Consider this instruction:-

1 Word = 16bit

Add

[2000]

[300]

Suppose this is our 3 word instruction, 

Opcode = 20bit, Address = 6bit each // So, in total 20 + 6 + 6 = 32;

Now, as we know that IR will be of 16 bit, i.e. it will only hold 16 bit of the instruction, now when it decode the instruction it will come to know that it is 3 word instruction, but where it will keep remaining 2 words of the instruction to complete the execution?

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First not evey processor support variable length instructions. Those who support like 8086 it use pre fetch queue. You can read more about it here https://en.m.wikipedia.org/wiki/Prefetch_input_queue

What about the the third questions, what does it mean? Anyone .

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PSW stands for program status word which stores the information about the state of the program.

It contain many registers like carry, zero, sign , overflow etc. Which show the state of the program. format and number of registers in PSW are variable depending on the needs.

Answer 1

Q1) How processor knows the instruction is one word or more?

at the time of decoding the cpu come to know whether the instruction is one word or more ...genrally store in Prefetched queue but its depends upon you whether your cpu is support queue or not bcz not all the cpu generally support it ..

 

Q2) Is it possible that opcode in instruction is greater than word size?

 multiple memory reference  are require to fetch the opcode only therefore this kind of condition is not in working ..

 
 

Q3) What is the format of PSW and how many PSW registers are there?

PSW stands for program status word which stores the information about the state of the program.

It contain many registers like carry, zero, sign , overflow et

Comments

PSW -program status word contain 2 things -(1)accumulator (2) flag register and both are used for processed data by ALU

flags are 2 kinds-

1. conditional flag( with reference of 8086 ,there are 6 flags-carry,overflow,parity,auxillary,zero,sign)
2. control flag(trap,interrupt,directional flag)