cs61c, Spring 1998
Midterm #1
Professor Clancy
Problem #1 (7 points, 14 minutes)
In lab assignment 4, you wrote a function that returned the contents of the various fields of a MIPS I-format instruction. In this problem we consider a similar task for the Prune 100 computer. The Prune, like the MIPS, has 32-bit instructions. The Prune has only 16 registers. In an I-format Prune instruction, the meaning
of the bits is as follows.
The first 8 bits are the op code.
The next 4 bits are the register to be modified by the instruction.
The last 20 bits are the immediate operand, in 1's complement.
Thus the equivalent to the MAL instruction, addi $10,-2 might appear in hexadecimal as
94 af ff fd
if the op code for the addi instruction were 94 base 16.
On the next page, write a MAL function SplitIFormat that returns the contents of the register and immediate fields of a Prune 100 I-format instruction. If written in C, its prototype would be
void SplitIFormat (int instr, int *register, int *immediate);
If written in C++, its prototype would be
void SplitIFormat (int instr, int ®ister, int &immediate);
Follow the conventions described in class and in lab and homework assignment 6 for passing arguments and managing registers and the system stadck. Provide comments sufficient for the graders to understand your work.
Problem #2 (7 points, 13 minutes)
Part a Translate the following MAL program segment to TAL. You may use either names or numbers for the registers.
li $t1,-5
loop: sub $t1,$t1,3
bgt $t1,$a1,loop
Equivalent TAL segment:
Part b
Your answer to part a should include a branch instruction. Translate this branch instruction to machine language by filling in the boxes below with 0's and 1's.
CS61c, Midterm #1, Spring 1998
cs61c, Spring 1998 Midterm #1 Professor Clancy 1
