---

How are the general purpose registers used in assembler programs ?

---

• Important fact about the computer system
  • Important fact that you need to recall about the memory to help you understand why we need registers:

    The computer memory can return some data to the CPU The computer memory can store some data from the CPU

    See: click here

    ---

    ---

  • Other than these 2 operations (read from memory and write to memory), the computer memory cannot perform any other operation....

    The memory cannot even transfer some data from one memory cell to another memory cell on its own (without the use of the CPU)

    In fact:

    If you want to copy some data from some memory cell to another memory cell, the data has to be read by the CPU first, and then the CPU will write the data to another memory cell !!!

  ---

  ---

  ---

  ---

• How to use the general purpose registers (of any CPU)
  • The general purpose registers are primarily used like a scratch pad to store values used in computations

    Because the ALU (Arithmetic and Logic Unit) is the computation circuit of the CPU and the ALU takes inputs from the general purpose registers, a rule of thumb in assembler programming is:

    When you want to perform computation on some value, you must first get that value in one of the general purpose registers !!! For example: In Java, you would write: z = x + y; when you want to add the values in x and y and store the sum in z This Java statement is excuted by the computer using the following assembler instructions (I give them now in English-description, later, we will learn how to write them in ARM assembler code): Get the value of x in register R0 Get the value of y in register R1 Add R0 and R1 and store result in R0 Put the value in R0 in variable z

  ---

  ---

  ---

  ---

• Review of mixed typed arithmetics
  • Recall that:

    In Java, integer typed operands (e.g.: byte, short and int) in an arithmetic expression are first converted to the int data type (and then the arithmetic operations are performed)

  ---

  ---

  ---

  ---

• Byte operands in general purpose registers
  • Recall that a general purpose register in the ARM processor consists of 4 bytes:

    ---

  • Recall that high level programming language has different data types, e.g.:

    byte (8 bites) <----- byte operands short (16 bits) int (32 bits)

    ---

    ---

  • How the ARM processor loads a byte typed value into registers to perform arithmetic/logic operations:

    The byte typed value is stored in the right most byte of the register Then the byte typed value is sign-extended to an int typed value Graphically explained:

    Later, I will show you assembler programming example.

  ---

  ---

  ---

  ---

• Short operands in general purpose registers
  • Recall that a general purpose register in the ARM processor consists of 4 bytes:

    ---

  • Recall that high level programming language has different data types, e.g.:

    byte (8 bits) short (16 bites) <----- short operands int (32 bits)

    ---

    ---

  • How the ARM processor loads a short typed value into registers to perform arithmetic/logic operations:

    The short typed value is stored in the right most 2 bytes of the register Then the short typed value is sign-extended to an int typed value Graphically explained:

    Later, I will show you assembler programming example.

  ---

  ---

  ---

  ---

• Int operands in general purpose registers
  • Because a register in the ARM processor is 4 bytes long, an int typed value is stored in a register "verbatim"

    I.e.: the processor does not need to perform any operation after storing the 4 bytes into the register.