1. Consider the following code used to implement the instruction
. sllv $s0, $s1, $s2
which uses the least significant 5 bits of the value in register $s2 to specify the amount register $s1 should be shifted left:
.data
.text
mask: .word 0xfffff83f
start: lw $t0, mask # load mask into $t0
lw $s0, shifter # load the instruction at label ‘‘shifter’’ into $s0 and $s0, $s0, $t0 # mask out the shift amount (making it zero)
andi $s2, $s2, 0x1f # mask out the least significant 5 bits of $s2
sll $s2, $s2, 6 # shift those bits left 6 places to align with shamt
or $s0, $s0, $s2 # or the value from $s2 into the shift inst’n as shamt sw $s0, shifter # store the modified inst’n back where we got it
shifter: sll $s0, $s1, 0 # execute the modified inst’n
Add comments to the code and write a paragraph describing how it works. Note that the two lw instructions are pseudoinstructions that use a label to specify a memory address that contains the word of data to be loaded. Why do you suppose that writing self-modifying code such as this is a bad idea (and oftentimes not actually allowed)?
2. The following MIPS instruction sequence could be used to implement a new instruction that has two register operands. Give the instruction a name and describe what it does. Note that register $t0 is being used as a temporary:
srl $s1, $s1, 1 #
sll $t0, $s0, 31 # These 4 instructions accomplish srl $s0, $s0, 1 # new ‘‘$s0 $s1’’
or $s1, $s1, $t0 #
3. Write a program in MIPS assembly language to convert an ASCII decimal string to an integer. Your program should expect register $a0 to hold the address of a null-terminated string containing some combination of the digits 0 through 9. Your program should compute the integer value equivalent to this string of digits, then place the number in register $v0. Your program need not handle negative numbers and need not be concerned about values larger than 231 − 1.
If a nondigit character appears anywhere in the string, your program should stop with the value -1 in register $v0. For example, if register $a0 points to a sequence of three bytes 5010 , 5210 , 010 (the null-terminated ASCII string 24), then when the program stops, register $v0 should contain the value
2410 , (or 11000 in binary). (The subscript ten means base 10.)
Submit your solution to question 3 on WebCT.
1
