# Addressing Mode - Intro to Computer Architecture - Homework, Exercises for Computer Architecture. Shree Ram Swarup College of Engineering & Management

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In the course of intro to computer architecture, the main points are:Addressing Mode, 16-Bit Number, 24-Bit Instruction, Auto-Increment Register-Indirect Addressing Mode, Speed Efficiency, Appropriate Register, Multiplic...
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Semantics: r1r0 � r1r0 

r3r2 where r1r0 means the 16-bit number formed by concatenating r1’s data

to r0’s data with the latter being the LS-byte and the

former

the MS-byte. (r2r3 is similarly defined).

2. (a) Design the format of a 24-bit instruction to perform a load word lw+ using the auto-increment

register-indirect addressing mode with the semantics ri  Mem  r j  ;r j  r j 

X . Note the initial (non-

incremented) value of r j is to be used for the address of the data word to be obtained from memory.

Clearly explain the format and what it means. Next, design the CU states for executing the lw+

instruction.

lw+ opcode = 010000

Hint: Just like in the sw instruction from Project 1, the 1st or least significant 2 bytes will have the

opcode and other information that the CU needs immediately to start processing the first part of the

instruction (ri  Mem  r j  ). The last (most significant) 8 bits of the instruction can be fetched after

the initial part is done and loaded into an appropriate register (mdr, ir0, ir1, etc.) so that it enables the

2nd part of the instruction (r j  r j 

X ) to be completed.

(b) Comment on the speed efficiency of using the 24-bit lw+ instruction above versus using two 16-bit

instructions load ind and add imm (you did these in Project 1) to accomplish the same effect as the lw+

instruction.

3. [mul slow ri r j rk] – type 1

Semantics: ri  r j  rk using the slow multiplication technique given below (repeated addition with

no shifting). Note that either of r j, rk could be negative.

mul slow opcode = 010001

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4. [div slow ri r j rk] – type 1

Semantics: ri  r j  rk using the slow division technique given below (repeated subtraction with no

shifting). Note that either of r j, rk could be negative.

div slow opcode = 010010

5. [div fast ri r j rk] – type 1

Semantics: ri   r j  rk using the fast non-restoring division technique (see lecture notes on “Iterative

Division”).

div fast opcode = 010011

For division, also assume that the D and V specified in r j, rk are unsigned integers represented in 7

bits (MSB–bit 7– is 0, while the numbers are represented in bits 6 to 0). This is needed in order to get

the negative V (for subtraction) in 2’s complement that is representable in 8 bits (this is automatically

done by choosing alu sel SUB so you don’t have to worry about it).

For the (slow) multiplication instruction use the following method for performing c a 

b:

1. c � 0; sign � 0

2. If b 0 then b � b /* convert b to a +ve # */

sign = not(sign) 

3. if (b == 0) then goto step 5 /* final result is in c */

else c c  a

4. b b 1; goto step 3.

5. if (sign=1) then c � c /* restore correct sign to the result */

For the slow division instruction use the following method for performing c � 

a 

b

(we assume b � 0):

1. c � 0; sign � 0

2. If b 0 then b � b /* convert b to a +ve # */

sign = not(sign) 

3. If a 0 then a � b /* convert a to a +ve # */

sign = not(sign) 

4. a a b

If (a 0) then goto step 6 /* quotient is in c */

else c c  1; goto step 4 

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5. if (sign=1) then c � c /* restore correct sign to the result */

Important Note: You should not change the values in the user registers (particularly those specified by the

r j and rk fields) specified by the instruction. You can first copy them into the scratch-pad registers (r4 to r6)

if their values need changing during the processing of the instruction.

Also, debug your design using sample instructions that you put in a .mem file.

On the date submit your debugged and tested control program along with the .mem fileswith your format

of the lw+ instruction inserted in the 24-bit place holders in the .mem file the we will provide you.

Theoretically analyze the maximum and the average # of cc’s for executing each instruction (assume

memory read/write takes 3 cc’s). Show your work clearly, and submit in a tabular form.

You will be given one or two machine language programs (.mem files) to test your C.U. design. Report

the initial and final states of the mythsim graphical interface by printing it out (with all register values etc.)

and the number of clock cycles required to execute each program.

The grading criterion will be similar to that in Project 1 60% of project weight

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