Operands

 Operands

i) Register Operands

·         Arithmetic instructions use register operands

   1       2      3      4

add   $s0, $s1, $s2           # f = g + h

1) operation by name

2) operand getting result (“destination”)

3) 1st operand for operation (“source1”)

4) 2nd operand for operation (“source2”)

·         MIPS has 32 x 32 bit register file (32 bit=1 word)

·         Syntax is rigid: 1 operator, 3 operands

–Why? Keep hardware simple via regularity

·         Register name

Register name	Number	usage
$zero	0	Constant  0
$at	1	reserved for assembler
$v0	2	expression evaluation and results of a function
$v1	3	expression evaluation and results of a function
$a0	4	Argument 1
$a1	5	Argument 2
$a2	6	Argument 3
$a3	7	Argument 4
$t0	8	temporary (not preserved across call)
$t1	9	temporary (not preserved across call)
$t2	10	temporary (not preserved across call)
$t3	11	temporary (not preserved across call)
$t4	12	temporary (not preserved across call)
$t5	13	temporary (not preserved across call)
$t6	14	temporary (not preserved across call)
$t7	15	temporary (not preserved across call)
$s0	16	saved temporary (preserved across call)
$s1	17	saved temporary (preserved across call)
$s2	18	saved temporary (preserved across call)
$s3	19	saved temporary (preserved across call)
$s4	20	saved temporary (preserved across call)
$s5	21	saved temporary (preserved across call)
$s6	22	saved temporary (preserved across call)
$s7	23	saved temporary (preserved across call)
$t8	24	temporary (not preserved across call)
$t9	25	temporary (not preserved across call)
$k0	26	reserved for OS kernel
$k1	27	reserved for OS kernel
$gp	28	pointer to global area
$sp	29	stack pointer
$fp	30	frame pointer
$ra	31	return address (used by function call)

·         Design principle : Smaller is faster

ii) Memory Operands

·         Data are stored in memory

·         “data transfer instructions”

–Transfer data between memory and registers

–Load lw: move data from memory to a register

–Store st: move data from a register to memory

·         Memory is byte addressed ( 1 byte = 8 bit)

·         Code for byte addressable memory

Original	Updated
a[12] = h + a[8] assume g, h = $s1, $s2 base address = $s3 lw  $t0 , 8($s3) add  $s1, $s2, $t0 sw 12($s3). $s1	a[12] = h + a[8] assume g, h = $s1, $s2 base address = $s3, word data lw  $t0, 32 ($s3) add $s1, $s2, $t0 sw  48($s3). $s1

iii) Immediate Operands or Constant

·         Small constants used frequently

e.g., A = A + 5;

B = B + 1;

·         Solutions? Why not?

– put 'typical constants' in memory and load them

– create hard-wired registers (like $zero) for constants

·         MIPS Instructions:

~ addi $29, $29, 4

~ ori   $29, $29, 4

why only “addi” and no “subi”

– use a negative constants          eg : addi  $s2. $s1, -1

·           Design Principle: Make the common case fast

KU MAN YI  B031210161