Compile a Strcpy Procedure

Compile a strcpy procedure (shows how to use strings) // x and y hold strings // each element is a char, ! only 1 byte long void strcpy (char x[], char y[]) { int i; i = 0; while ((x[i] = y[i]) != ‘\0’) i += 1; } x[0] ! $a, y[0] ! $a1 i ! $s0 Adjust the stack pointer and save $s0 (as i), the only register to be used. strcpy: addi $sp, $sp, -4 sw $s0, 0($sp) Initialize $s0 to 0 (i = 0) addi $s0, $zero, $zero Need to copy y[i] into x[i]. add i to y[0]!s address L1: add $t1, $s0, $a1 Note: just adding 0 to base address instead of 4 bcos we are dealing with bytes now (each element of x[] is a char of a byte): Load the character y[i] in $t2 lb $t2, 0($t1) Similarly load address of x[i] into $t3 add $t3, $s0, $a0 Store the byte from y[i] into x[i]: sb $t2, 0($t3) # x[i] = y[i] Exit if character is 0 beq $t2, $zero, L2 # if y[i]=0 go to L2 Otherwise, increment i and loop back addi $s0, $s0, 1 j L1 If done, restore $s0 L2: lw $s0, 0($sp) addi $sp, $sp, 4 jr $ra Logical Operations Shifts • shift left & right (C ver.: << , >> ) • shifts a register!s value by given amount to left, stores zeros on the right, and stores the new value in new reg. • similarly srl (shift right logical) # $t2 = ($s0 shifted left by 4 bits) • sll $t2, $s0, 4 • the shamt in the R-format is used for this op rs rt rd shamt funct 0 0 16 10 4 0 • sll encoded by both op and funct • rd ! t2 • rt ! s0 • rs is unused, so 0 • A bonus • shift left by i == multiply by 2i Other Operations • and $s0, $t1, $t2 • or $s0, $t1, $t2 • nor $s0, $t1, $t2 • andi $s0, $t0, 100 # # # # AND of t1 and t2 OR of t1 and t2 NOR of t1 and t2 AND of t0 and 100