Question about lb instruction

[Ryan Tio]

# Lecture Question

Week : 3

Slide deck name: L06 - Instructions

Question: On the final slide of the instructions video, I'm not sure what the 3(x11) in lb x10, 3(x11) means. Is it moving 3 bytes from the starting point of x11, or is it meaning to copy the lowest 3 bytes of x11? I'm actually quite iffy on the whole lb sb concept as a whole.

We pass the src parameter as a memory address, and it will only copy the single byte at that address then sign extend to fill the rest of the register?

[Arrvindh Shriraman]

I'm not sure what the 3(x11) in lb x10, 3(x11) means. Is it moving 3 bytes from the starting point of x11, or is it meaning to copy the lowest 3 bytes of x11?

Neither :)

There are 3 parts to the instruction. 

lb - I want to load a byte from memory

x10 - When I read the byte from memory which register should I copy the value to

x11 - Base pointer

3 - An immediate value that is added to x11. 

     Immediate means the binary representation of number is included as part of the instruction itself. 

     

It means treat x11 as a pointer. Add 3 to the pointer address. E.g., lets say x11 is 0x10. The new pointer address would be 0x13.

Read 1 byte starting from 0x13 in memory and copy it to x10.

x10 = mem[x11+3]

https://www.cs.sfu.ca/~ashriram/Courses/CS295/assets/notebooks/RISCV/RISCV_CARD.pdf  

includes more details (however it also included discussion on instruction encoding which is discussed only in Week 4).

On Monday, September 21, 2020 at 3:15:13 PM UTC-7, Ryan Tio wrote:

# Lecture Question

Week : 3

Slide deck name: L06 - Instructions

Question: On the final slide of the instructions video,  I'm actually quite iffy on the whole lb sb concept as a whole.