Computer Language : Data Transfer Instruction

Data Transfer Instruction

·         Load: lw copy data from memory to registers

·         Store: sw copy data from registers to memory

Data transfer	load word	lw $1,(100)$2	$1 = Memory[$2+100]	Data from memory to register
	store word	sw $1,(100)$2	Memory[$2+100] = $1	Data from memory to register
	load upper immediate	lui $1,100	$1 = 100 * 216	Load constant in upper 16bits

·         To transfer a word of data, need to specify two things:

–Register: specify this by number (0 - 31)

–Memory address:

1.       Think of memory as a 1D array

2.       Address it by supplying a pointer to a memory address

3.       Offset (in bytes) from this pointer

4.       The desired memory address is the sum of these two values, e.g., 8($t0)

5.       Specifies the memory address pointed to by the value in $t0, plus 8 bytes (why “bytes”, not “words”?)

6.       Each address is 32 bits

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i.) Data Transfer : from memory to register

·         Load Instruction Syntax:

1    2     3   4

lw $t0,12($s0)

1) operation name

2) register that will receive value

3) numerical offset in bytes

4) register containing pointer to memory

·         Example: lw $t0,12($s0)

 – lw (Load word, so a word (32 bits) is loaded at a time)

– Take the pointer in $s0, add 12 bytes to it, and then load the value from the

--memory pointed to by this calculated sum into register $t0

Notes:

– $s0 is called the base register, 12 is called the offset

– Offset is generally used in accessing elements of array: base register points to the beginning of the array

iii.) Data Transfer : from register to memory

·         Example:     sw $t0,12($s0)

–sw (meaning Store word, so 32 bits or one word are loaded at a time)

–This instruction will take the pointer in $s0, add 12 bytes to it, and then store the value from register $t0 into the memory address pointed to by the calculated sum.

By : Lua Xin Lin     B031210345