Carl Hamacher

Question

Registers R1 and R2 of a computer contain the decimal values 1200 and 4600.What is the Effective
address of the memory operand in each of the following instructions?
a) Load 20(R1),R5
b) Move #3000,R5
c) Store R5,30(R1,R2)
d) Add -(R2),R5
e) Subtract (R1)+,R5

Comments

instruction format of LOAD and STORE

Load destination,source

Store source,destination

@Bikram sir please varify.

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@Reena There is no such rule- it depends on a given architecture. But when instructions are given we can deduce the source/destination by identifying which is a memory location.

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1) effective ddress woould be 20 + 1200

2)effective address would be just R5 as it is an immideate operand

3)20 + 1200 + 4600

is this right?

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@reena_kandari

here

Load source ,destination

Store destination, source

you said it in reverse..

Answer 1

Registers R1 and R2 of a computer contain the decimal values 1200 and 4600, we have to find effective adress of associated memory operand in each instruction:

 Load 20(R1),R5 : This means load 20+R1 into R5 .   R1= 1200 , R1 + 20 = 1220 , so R5 have 1220 , Effective address of R5 is 1220.

 Move #3000,R5 : This means move value 3000 into R5 , so effective address is part of the instruction whose value is 3000.

Now R5 = 3000

 Store R5,30(R1,R2) : This means 30+R1+R2 and store the result into R5 .

so R5 = 30+1200+4600 =  5830 , so now R5 value is 5830 , the effective address is 5830.

 Add -(R2),R5 : This means -1 from  R2 value and store the result into R5 . So R5= 4600 - 1 = 4599 , effective address of R5 is 4599 .  It is pre decrement addressing.

 Subtract (R1)+,R5 : This means effective address is contents of R1 so EA = 1200 .

It is post increment addressing .

Effective addresses are

1. 1220
2. 3000 [ it is not the effective address , it is the address of the instruction part where 3000 is stored ] 
3. 5830
4. 4599
5. 1200

Comments

@Bikram sir

The result should be stored in R5,  not R1 sir.

I didn't have any problem with the calculation of EA.  I just got confused about the meaning of the instruction following your interpretation,  but now everything got cleared.  Thanks.

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@ Pinaki Dash  

See what this instruction Subtract (R1)+,R5  means :

It means first subtract R5 from R1 then store that result into R1 then increase R1 value  by word length .

so M[R1] <- M[R1] - R5 then we increase R1 value word by word. It is post increment.

If I say Add R1, R2 Then the register contains the operand. It simply add R1 with R2.

If I say Add  (R1), R2 : Then there is one level of indirection for R1 i.e. R1 doesn't contain operand directly but address of operand. so (R1) is Mem[R1] . 

This question asks what is effective address of the memory operand , that memory addres is value of R1 only which is our Effective address here .

Similarly, Add -(R2),R5 means first decrese value of R2 by 1 then store it to R5 . It is pre decrement so we store it in R5.

But Substract (R1)+,R5 is post increment hence result first store in R1 then increse word by word .

Hope you get my point.

An effective address is the value which is used by a fetch or store operation to specify which memory location is to be accessed by the operation from the perspective of the entity which issue the operation .

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3000 is not the EA, the address of the instruction part where 3000 is stored is the EA.