Im learning MIPS in MARS and i've been trying to comprehend the lbu instruction, I need some help unwrapping concepts. In the "Help" section, lbu is defined as:

"lbu $t1,-100($t2)"

"Load byte unsigned : Set $t1 to zero-extended 8bit value from effective memory byte address"

1. What does "-100($t2)" do?
2. What does zero-extended 8bit value mean exactly?
3. Am I understanding correctly that the lbu instruction copies the first (rightmost) 8 bits from a register and inserts it to into another register?
4. If so, does the "effective memory byte address" reffer to the origin register (from which the copy is made)?

I can't find any good explanation on this. Perhaps I made my questions a bit convoluted but hopefully you can get the gist. Thanks in advance.

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2 Answers

In "lbu $t1,-100($t2)" $t1 and $t2 are placholders for any of the 32 MIPS registers, and -100 is a placeholder for a 16 bit sign-extended immediate.

The instruction reads a byte from the effective memory address result from adding the contents of the second register ($t2 in the example) and the 16 bit signed immediate, then the contents of the target register ($t1 in the example) are loaded with the byte read from memory (in the 8 least significant bits) and filled with 0s in the 24 most significant bits.

So for example you may have the following sample code in MARS:

 .data
info: .byte 0x10 .byte 0x20
last: .byte 0x30 .text
li $a0, 0xFFFFFFFF
la $t0, info
lbu $a0, 0($t0)
la $t1, last
lbu $a1, -1($t1)
li $v0, 10
syscall # terminate

Before executing lbu $a0, 0($t0) $a0 will hold 0xFFFFFFFF and $t0 will hold the address where the data section begins.

After that instruction is executed $a0 will hold 0x10 that is the content of the first byte of the data section.

Before executing lbu $a1, -1($t1) $t1 will hold the address where the data section contains the last (third) byte (the address whose contents are 0x30 in the example). Note also that I am using -1 as the immediate in this instruction.

After that instruction is executed $a1 will hold 0x20 that is the content of the second byte of the data section.

2

I found a MIPS assembly reference here:

It says ($t2) is the address held in register $t2 (one of the register set $8 to $15), and -100 means subtract 100 and use that as the actual address.