Assembly Variables : Registers and Arithmetic Operations

Assembly Variables : Registers

Unlike high level language like C++ or Java,assembly cannot use variable because to keep hardware simple.

- Assembly Operands are Registers
                - Limited number of special locations built directly into the hardware.
                - Operation can only be performed on Registers.

- Benefit
                1) Since registers are directly in hardware,they are very fast(faster than 1 bilionth of a
                      second).

- Drawback
                1) Since registers are in hardware,there are a predetermined number of them.
                               - Solution : MIPS code must be very carefully put together to efficiently use                                                             registers.

- There are 32 registers in MIPS,because smaller is faster.Each MIPS registers is 32 bits wide.Groups          of 32 bits called a word in MIPS.

Arithmetic Operations

- Add and subtract, three operands.
                - Two sources and one destinations.

- All arithmetic operations have this form

- Design Principle 1 : Simplicity Favours regularity
                1) Regularity makes implementation simpler.
                2) Simplicity enables higher performance at lower cost.

- Registers are numbered from 0 to 31

- Each register can be referred to by number or name.

- $0,$1,$2….until $31

- By convention,each register also has a name to make it easier to code.

- In Assembly Language, The register have no type.Operation determines how register contents are treated.

- Comments in assembly,hash(#) is used for MIPS comments
                1) anything from hash mark to end of line is a comment an will be ignored.

- In assembly language,each statement, excutes exactly one of a short list of simple commands.

MIPS Addition and Subtraction

- Syntax of Instructions:

1  2,3,4

where :

1) Operation by name.
2)Operand getting result(destination).
3)1^st Operand for operation(source 1).
4) 2^nd Operand for operation(soruce 2).

- Syntax is rigid
                1) 1 operator , 3 operands.
                2) Because to keep hardware simple via regularity.

- Addition in Assembly
                1) example: add  $s0, $s1, $s2(in MIPS).
                2) Equivalent to: a = b + c(in C).
                3)Where MIPS registers $s0, $s1, $s2 are associated with C variables a,b,c.

- Subtraction in Assembly
                1) example: sub  $s3, $s4, $s5(in MIPS).
                2) Equivalent to: d = e - f(in C).
                3)Where MIPS registers $s3, $s4, $s5 are associated with C variables d,e,f.