# Pseudo instructions Pseudo instructions MIPS supports pseudo instructions

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Pseudo instructions

Pseudo instructions • MIPS supports pseudo instructions. We have seen some like – li $t 0, 4 which set $t 0 to 4. – la $t 0, A which puts the address of label A (a 32 -bit value) into $t 0. – bgt $t 0, $t 1, L 1 which goes to L 1 if $t 0 > $t 1

Pseudo instructions • Pseudo instructions are not real instructions implemented in hardware. They are created to make the program more readable. • A pseudo instruction usually (not always) maps to several real instructions. The mapping is one-to-one.

Pseudo instructions • For example, li $t 0, 4 translate to ori $t 0, $0, 4 but what should li $t 0, 90000 translate to?

Pseudo instructions • So li $t 0, 90000 translates to lui ori $1, 1 #load upper 16 bits $t 0, $1, 24464 • The special register $1 is $at and should only be used for pseudo instructions.

Pseudo instructions • How to translate ``lw $t 0, val’’ ?

MIPS mul div, and MIPS floating point instructions

Multiply and Division Instructions • mul rd, rs, rt – put the result of rs times rt in rd • div rd, rs, rt – A pseudo instruction – put the quotient of rs/rt into rd

hi and lo • mult rs, rt – put the high word in hi and low word in lo. • div rs, rt – put the remainder in hi and quotient in lo.

Load and Store • Load or store from a memory location (pseudoinstruction ). Just load the 32 bits into the register. – l. s $f 0, val – s. s $f 0, val • Load immediate number (pseudoinstruction ) – li. s $f 0, 0. 5

Print and Read • Print: – li $v 0, 2 – li. s $f 12, 0. 5 – syscall • Read – li $v 0, 6 – syscall – (the read will be in $f 0)

Arithmetic Instructions • • • abs. s add. s sub. s mul. s div. s neg. s $f 0, $f 0, $f 1, $f 1 $f 2

Data move • mov. s $f 0, $f 1 copy $f 1 to $f 0. • mfc 1 $t 0, $f 0 copy $f 0 to $t 0. • mtc 1 $t 0, $f 0 copy $t 0 to $f 0.

Convert to integer and from integer • cvt. s. w $f 0, $f 1 – convert the 32 bit in $f 1 currently representing an integer to float of the same value and store in $f 0 • cvt. w. s $f 0, $f 1 – the reverse

Comparison instructions • c. lt. s $f 0, $f 1 – set a flag in coprocessor 1 if $f 0 < $f 1, else clear it. The flag will stay until set or cleared next time • c. le. s $f 0, $f 1 – set flag if $f 0 <= $f 1, else clear it • bc 1 t L 1 – branch to L 1 if the flag is set • bc 1 f L 1 – branch to L 1 if the flag is 0

Computing the square root of a number n • The Newton’s method x’=(x+n/x)/2 – For any n, guess an initial value of x as the sqrt of n and keep on updating x until is the difference between the two updates are very close. – The idea is that x’=x-f(x)/f’(x), where f(x) is x 2 -n=0.

val 1: val 2: . data. float 0. 6. float 0. 8 msg_done: . asciiz "donen". text. globl main: mtc 1 $a 0, $f 0 li. s $f 20, 2. 0 li. s $f 21, 0. 001 comparision div. s $f 1, $f 0, $f 20 # $f 1 gets n/2 calsqrtloop: div. s $f 2, $f 0, $f 1 add. s $f 2, $f 1 div. s $f 2, $f 20 sub. s $f 3, $f 2, $f 1 abs. s $f 3, $f 3 c. lt. s $f 3, $f 21 bc 1 t calsqrtdone mov. s $f 1, $f 2 j calsqrtloop li. s $f 0, 361. 0 mfc 1 $a 0, $f 0 jal calsqrt done: mtc 1 $v 0, $f 12 li $v 0, 2 syscall eixt: li $v 0, 10 syscall # # calsqrtdone: calsqrt: calculating the square root of n using the formular x'=(x+n/x)/2 loop until |x'-x| < 0. 001 calsqrt: addi swc 1 swc 1 $sp, -24 $f 0, 20($sp) $f 1, 16($sp) $f 2, 12($sp) $f 3, 8($sp) $f 20, 4($sp) $f 21, 0($sp) # $f 0 gets n # $f 20 storing constant 2 for dividing # $f 21 storing constant 0. 001 for exit mfc 1 $v 0, $f 2 lwc 1 lwc 1 addi $f 0, 20($sp) $f 1, 16($sp) $f 2, 12($sp) $f 3, 8($sp) $f 20, 4($sp) $f 21, 0($sp) $sp, 24 jr $ra # # # $f 2 $f 3 set gets n/x + x gets x'=(n/x + x)/2 gets x'-x gets |x'-x| the flag if |x'-x| < 0. 001

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