A byte
There are two instructions that load a byte from memory. The address of the byte is calculated at run time by adding an offset to a base register (just as with the load word and store word instructions). The instructions differ in how the 8-bit byte is put into the 32-bit register.
lb t,off(b) # $t ← Sign-extended byte # from memory address b+off # b is a base register. # off is 16-bit two's complement.
The lb instruction
loads the byte from memory into the low order eight bits
of the register.
These are bits 0-7 of the register.
Then it
copies bit 7 to bits 8-31 of the register
(all bits to the left of bit 7).
Another way to say this is that the lb instruction
loads the register with a 32-bit sign extended version of
the byte at the designated address.
The address in memory does not have any alignment restrictions.
Use
the lb instruction when the byte is regarded as an
8-bit signed integer in the range -128...+127 and you want
a 32-bit version of the same integer.
The lbu instruction
fills bits 8-31 of the register with zeros.
Use this instruction when the byte is regarded as a
ascii character or 8-bit unsigned integer.
Another way to say this is that the lbu instruction
loads the register with a 32-bit zero extended version of
the byte at the designated address.
lbu t,off(b) # $t ← Zero-extended byte # from memory address b+off # b is a base register. # off is 16-bit two's complement.
The address in memory does not have any alignment restrictions.