Recap:

• mov
  • Transfer data from one register to another
• movia
  • Setting a register to a known, specific memory address
  • Used for initialization or to set up pointers
• ld/ldw
  • Used to read data from memory and place it into a register
  • ldw r1, 0(r2): Loads a word from the memory address contained in r2 into register r1
  • Moving data from memory to a register
• st/stw
  • Used to write data from a register to a memory location
  • stw r1, 0(r2): Stores the word from register r1 to the memory address contained in r2
  • Moving data from a CPU register to memory

Accessing Half-words and Bytes

• So far all data coming from/to memory has been 32 bits in size
• When there is a lot to store, it really matters if we can store a given number in fewer bits
• Recall ldw r9, (r10)
  • Loads a 32-bit word from memory into r9
  • (r10) indicates that the source of data is the memory address contained in register r10
• We can also load 16-bits (half-word) using ldh or a byte (8-bits) using ldb
• ldb r10, (r12)
  • Assuming r12 contains the address of the byte we want from memory
  • This instruction loads that byte from memory into r10
  • Into the low-order bits r10 (7-0)
  • The upper bits r10 (31-8) are set to 0
• stb r8, (r9)
  • Stores (puts into memory) bits r8 (7-0, the LSB of r8) into the byte at the address given by r9
• In assembly language programs, you’ll need to reserve space in memory for bytes and half-words
• For reserving a byte: smallthing: .byte 50
  • Reserves 1 byte in memory (1 address)
  • Sets smallthing equal to that address
  • 50 must fit into 8 bits
• Similarly, half words: onehalfthing: .hword 0x660

Some rules:

• The address of words in memory must be divisible by 4 (aligned)
• The address of half words in memory must be divisible by 2
• All addresses can be byte addresses