ex1 solns

							Computer Systems and Architecture Exercise Sheet 1: Instruction Sets
1. Some simple arithmetic Show how the following code fragments could be written in MIPS assembly language, assuming that all variables are stored in main memory. You should only need to use the following instructions (see the attached list for full details): lw, dw, add, sub, addi, subi, mult a) b) c) d) e) a a a a a = = = = = (b + c) - (d + e); b * ( c + d); b*c + 25; (b + 10)*c – (d – 5); (b + 5)*c + (b – 3);

Solutions: Possible implementations of these code fragments in MIPS assembly language are: a) lw $r1 , &b lw $r2 , &c add $r3 , $r1 , $r2 lw $r1 , &d lw $r2 , &e add $r4 , $r1 , $r2 sub $r1 , $r3 , $r4 sw $r1 , &a b) lw $r1 , &b lw $r2 , &c lw $r3 , &d add $r4 , $r2 , $r3 mult $r2 , $r1 , $r4 sw $r2 , &a c) lw $r1 , lw $r2 , mult $r3 addi $r1 sw $r1 , &b &c , $r1 , r2 , $r3 , 25 &a // // // // // // // // // // // // // // // // // // // // // // // // // // // // // // // // // // Load b into r1 Load c into r2 Add b and c, result Load d into r1 Load e into r2 Add d and e, result Do the subtraction, Store the result in

in r3

in r4 result in r1 a

Load b into r1 Load c into r2 Load d into r3 Add c and d, result into r4 Do the multiplication, result in r2 Store the result in a Load b into r1 Load c into r2 Do a*c, result into r3 Add 25 to a*c using an immediate add Store the result in a Load b into r1 Add 10 to b, result in r2 Load c into r1 Do (b+10)*c, result into r3 Load d into r1 Subtract 5 from d, result into r2 Do the subtraction, result into r1 Store the result in a Load b into r1 Add 5 to b, result in r2 Load c into r3 (still need b in r1) Do (b+5)*c, result into r4 Subtract 3 from b, result into r2 Do the addition, result into r1 Store the result in a

d) lw $r1 , &b addi $r2 , $r1 , 10 lw $r1 , &c mult $r3 , $r2 , $r1 lw $r1 , &d subi $r2 , $r1 , 5 sub $r1 , $r3 , $r2 sw $r1 , &a e) lw $r1 , &b addi $r2 , $r1 , 5 lw $r3 , &c mult $r4 , $r2 , $r3 subi $r2 , $r1 , 3 add $r1 , $r2 , $r4 sw $r1 , &a

[Note: the main challenge in exercise (e) is to make sure that you only have to load b once]

2. Doing things the other way around Given the following fragments of MIPS assembly code, write the equivalent high-level construct a) lw $r1 , &b lw $r2 , &c sub $r3 , $r1 , $r2 lw $r1 , &d lw $r2 , &e sub $r4 , $r1 , $r2 add $r1 , $r3 , $r4 sw $r1 , &a b) lw $r1 , lw $r2 , lw $r3 , addi $r3 mult $r1 mult $r1 sw $r1 , c) &b &c &d , $r3 , 6 , $r1 , $r2 , $r1 , $r3 &a

lw $r1 , &b addi $r2 , $r0 , 10 mult $r1 , $r1 , $r2 sw $r1 , &a   d)  lw $r1 , &a addi $r1 , $r1 , 1 sw $r1 , &a   e) lw $r1 , &b mult $r1 , $r1 , $r1 lw $r2 , &c subi $r2 , $r2 , 3 add $r1 , $r1 , $r2 sw $r1 , &a Solutions: a) b) c) d) e) a a a a a = = = = = (b – c) + (d – e); b*c*(d+6); b*10; a + 1; b^2 + c – 3;

// Hint: r0 is always zero.