CODEWITHSUNDEEP
assemblymips
nihulus
nihulus

Reputation: 1495

MIPS: load byte instruction

Assume that the processor is executing the following instruction:

lb $t3,-7($s0)

Moreover, assume that the instruction above is located at address 0x004000f0, register $t3 contains value 0x20040144, and that register $s0 contains value 0x305502db. What value has registers $t3 after that the instruction has finished executing?

From my calculation:

The format of the instruction is:

lb RegDest, Offset(RegSource)

we know $s0 contains value 0x305502db, which leads to 0x305502db - 7 = 0x305502d4 and this value will be loaded in $t3. I am confused here now because the correct answer says $t3 = unknown, how come?

Upvotes: 0

Views: 1843

Answers (1)

Martin Rosenau
Martin Rosenau

Reputation: 18523

0x305502db - 7 = 0x305502d4

... so the lb instruction will load the byte from the memory (for example: RAM) at address 0x305502d4, sign-extend it and write it into the register $t3.

If the RAM contains the value 0x5A at address 0x305502d4, $t3 will contain the value 0x0000005A. If it contains the value 0xA5, $t3 will contain the value 0xFFFFFFA5.

... the correct answer says $t3 = unknown, how come?

Your exercise does not contain any information about the content of the RAM at address 0x305502d4.

Therefore you cannot say which value $t3 will contain.

Upvotes: 3

Related Questions