Lecture Microcomputer principles and applications - Chapter 3: Assembly language programming

Contents: Introduction to Assembly, Instruction Cycle, Addressing Modes

Chapter 3

Assembly language

programming

3.1 Introduction to Assembly

High level vs Assembly

High level languages Assembly language

• More programmer friendly • Lower level, closer to ISA

• More ISA independent • Very ISA-dependent

• Each high-level statement

translates to several instructions

in the ISA of the computer

• Each instruction specifies a single ISA instruction

• Makes low level programming more user friendly

• More efficient code

3.1 Introduction to Assembly

Assembler syntax

{label[:]} mnemonic {operand list} {;comment}

• Symbols:

⋄ Used as labels, constants, and substitution values and

stored in a symbol table

⋄ A symbol name is a string of up to 200 alphanumeric

characters (A-Z, a-z, 0-9, $, and _), cannot contain embedded blanks, is case sensitive

⋄ The first character cannot be a number

• Labels:

⋄ Labels are symbols

3.1 Introduction to Assembly

Assembler syntax

{label[:]} mnemonic {operand list} {;comment}

• Labels:

⋄ Begined in column 1 and is optionally followed by a colon

⋄ The value of a label is the current value of the Location

Counter (address within program)

⋄ A label on a line by itself is a valid statement

⋄ Labels used locally within a file must be unique

• Mnemonics:

⋄ Cannot start in column 1 If it does, it is interpreted as a label

3.1 Introduction to Assembly

Assembler syntax

{label[:]} mnemonic {operand list} {;comment}

• Mnemonics:

⋄ Contains one of the following items: Instruction, Assembler

directive, Macro directive, Macro invocation

⋄ A label on a line by itself is a valid statement

⋄ Labels used locally within a file must be unique

• Operands:

⋄ Contains one or more operands

⋄ An operand may consist of: symbols, constants,

expressions

⋄ Operands are separated with commas

3.2 Instruction Cycle

• Instruction Fetch (Get what you need to do)

• Instruction Decode (Understand what you need to do)

• First Operand Fetch (Not enough information, get some more)

• Second Operand Fetch (Still not enough information, get some

more)

• Execute (Do it !)

• Write back (Write result of the operation)

3.3 Addressing Modes

Source Addressing Modes

• Register

• Indexed

• Symbolic (PC Relative)

• Absolute Address

• Indirect Register

• Indirect Auto-increment

• Immediate

3.3 Addressing Modes

Destination Addressing Modes

• Register

• Symbolic (PC Relative)

• Absolute Address

• Indexed

3.3 Addressing Modes

3.3.1 Register Mode

Example:

mov R5, R6

Explanation:

Moves the content or the register R5 into R6 without altering R5

Usefulness

Save a register to another

3.3 Addressing Modes

3.3.2 Indexed Mode

Example

mov 4(R5), R6

Explanation

• Add 4 to the content of R5 inside the CPU

• Fetch the memory address from the forementionned

computation

• Store the value into R6

Usefulness

Access an item in memory (eg an array) with a constant offset

3.3 Addressing Modes

3.3.3 Symbolic Mode

Example

mov 0x1234, R6

explanation

• Add 0x1234 to the PC to generate the address

• Fetch the memory from the address of the forementionned

computation

• Store the value into R6

Usefulness

Access an array of data stored in the program memory

3.3 Addressing Modes

3.3.4 Absolute Mode

Example

mov &0xDEAD, R6

Explanation

• Fetch the memory from the address 0xDEAD

• Store the value into R6

Usefulness

Access memory at a known address (eg Peripheral)

3.3 Addressing Modes

3.3.5 Indirect Register Mode

Example

mov @R8, R6

Explanation

• Fetch the memory at the address contained in R8

• Store the value into R6

Usefulness

Use a register as a pointer to memory

3.3 Addressing Modes

3.3.6 Indirect Autoincrement Mode

Example

mov @R8+, R6

Explanation

• Fetch the memory at the address contained in R8

• Store the value into R6

• Increment R8

Usefulness

• Copy a data to somewhere else in 1 instruction

• Stack Popping

3.3 Addressing Modes

3.3.7 Immediate Mode

Example

mov #0xBEEF, R6

Explanation

Load R6 with 0xBEEF

Usefulness

Initialize a register with a value