Department of Computer Science
COMP2120 Computer Organization
Assignment 4
Due Date: Apr 30, 11pm, 2025.
Figure 1: A simplified CPU
This assignment is based on the CPU and simulator in Assignment 2.
In this assignment, extra instructions are added. They are the PUSH, POP, CALL and RET instruction. In order to implement these instructions, the CPU is modified as follows:
1. A new register (SP, the stack pointer) is included. SP provides output to S1-bus, and receives input from D-bus. Also, the SP has special hardware to increase and decrease its value by 4 (similar to PC). This is provided by the special function do incSP(), and do decSP(), which is in turn controlled by the flag incSP and decSP.
2. A new register (TEMP) is included, which is directly connected to the MAR only, via a dedicated data path. Again you can move data between MAR and TEMP and special function do MAR to TEMP() and do TEMP to MAR() are provided, which are controlled by the MAR to TEMP and TEMP to MAR flag.
3. A new flag push pop is included, which will move the SP to MAR. Otherwise, the CPU remains the same.
New instructions provided include:
PUSH Rn : SP ← SP-4; mem[SP] ← Rn
|
00001010
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n
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00000000
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00000000
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POP Rn : Rn ← mem[SP]; SP ← SP+4
|
00001011
|
00000000
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00000000
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n
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CALL proc :
|
00001100
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00000000
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11111111
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00000000
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RET :
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00001101
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00000000
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00000000
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00000000
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Summary Opcode:
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Instruction
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Opcode
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Instruction
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Opcode
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Instruction
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Opcode
|
|
ADD
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00000000
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MOV
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00000101
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PUSH
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00001010
|
|
SUB
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00000001
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LD
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00000110
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POP
|
00001011
|
|
NOT
|
00000010
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ST
|
00000111
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CALL
|
00001100
|
|
AND
|
00000011
|
Bcc
|
00001000
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RET
|
00001101
|
|
OR
|
00000100
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HLT
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00001001
|
|
The program
The revised simulator program is given in sim2. py. Study the simulator code carefully.
1. Hand assemble the following assembly code and put it in a program file. Run the simulator on this program. Explain what the function SQ does?
/* Procedure to calculate , input is R10, output is R11 */
/* The proc uses R12 and R13, need to save them on entry */
/* and restore them when exit*/
2. Run the simulator in debug mode. Write down the data transfer/transformation sequences involved in the execution of the instructions CALL and RET.
You may skip intermediate step provided by the simulator, for example the in- struction fetchs step should look like:
MAR <- PC
IR <- mem[MAR]
or in English, move the value of PC to MAR. Then read memory and the result (mem[MAR]) is moved to IR, i.e. just write down the source and destination of the data movement, without the paths etc.
3. Modify the program so that it will calculate the value of 1 — 2 + 3 — 4 · · · — 8 + 9.
That is,
sum = 0;
for i=1 to 9 do sum += sq(i)
Where sq(i) return i when i is odd, otherwise return -i. Note that the original program is already a loop from 1 to 9. Just replace the function SQ by
if (R10 is odd) R11 = R10;
else R11 = 0 - R10;
Since we don’t have a NEG instruction, to find —x, we use 0 — x.
To check if a number x is odd, just check if the rightmost bit is 1. We can find x AND 00000000 ··· 0001. (i.e. 1) After AND operation, all bits ANDed with 0 will be 0. If the rightmost bit is 0, then the result is 0. Otherwise the result is non-zero.
Note that the address of P1, P2 and P may got changed when the length of the function SQ is changed. You may need to change the address of them in the program, e.g. in line 2
LD P1,R1
you may need to find the new address of P1, and also in line 4 ··· .