Assembly Language and Machine Code

Assembling By Hand

• An assembler translates assembly language into machine code.
• A disassembler does the opposite, translating machine code into assembly language.
• When submitting ARMlite code, it is assembled into machine code and stored in memory as binary, hexadecimal, or decimal.
• Each line of assembly code is translated into machine language.
• Machine code is the executable form of a computer program and is often represented in files with extensions like .obj or .exe.
• Assembly language is a symbolic representation of machine code, making it more readable and understandable.

Language Levels

• High-level languages are designed to be readable and writable by humans.
• Assembly language is a lower-level language that is closer to the machine's instruction set.
• Machine language is the lowest level of programming and is directly understood by the computer's hardware.
• Compilers translate high-level language programs into assembly or machine code.

Compiler vs. Assembler

• Program faults caused by compiler errors are difficult to identify and fix.
• Compiler developers invest significant effort to ensure the accuracy of their software.
• Compilers translate high-level languages into assembly or machine code.
• Assemblers translate assembly language into machine code.
• Assemblers and disassemblers work with assembly language and machine code, while compilers work with high-level languages. ### Assembling machine code
• The ARM instruction format is:
  • Opcode{conditions}{flags} Rd, Rn, operand2
• {cond} is an optional two-letter condition, e.g. EQ
• {flags} is an optional additional flag, e.g. S
• Rd is the destination register
• Rn is the first source register
• Operand2 is the second register or Rn if there is only one operand
• Example:
  • MOV R2, R1
  • The condition is always, so Cond is 00
  • I is 0 for immediate, and 1 for register
  • Opcode for MOV is 1101
  • S is 0, as there is no condition code set
  • Rn is 0000, there is no second source register
  • Rd is 0010 (R2)
  • Operand2 is 000000000001 (R1)
  • This translates to: 1110 0001 1010 0000 0010 0000 0000 0001
• Assembling a more complicated example:
  • ADD R3, R2, R1
  • The condition is always, so Cond is 00
  • I is 1 for register
  • Opcode for ADD is 0100
  • S is 0
  • Rn is 0010 (R2)
  • Rd is 0011 (R3)
  • Operand2 is 000000000001 (R1)
  • This translates to: 1110 0000 1000 0010 0011 0000 0000 0001

### Disassembling machine code

• To disassemble machine code by hand you need to refer to the ARM data sheet
• Example of disassembling hexadecimal code:
  • 0xE3A02003
  • Cond is 00
  • I is 1
  • Opcode is 1101
  • S is 0
  • Rn is 0000
  • Rd is 0010 (R2)
  • Operand2 is 000000000011 (R3)
  • This translates to: MOV R2, #3
• Explanation of the example:
  • Condition is Always
  • Opcode is MOV
  • S is 0, so the condition codes are not used
  • Rn is N/A as there is no second register
  • Rd is R2
  • Operand2 is 3
  • Therefore the machine code disassembles to: MOV R2, #3

Explore the intricacies of assembly language, machine code, and the role of assemblers and disassemblers. This quiz covers the differences between high-level programming languages and their lower-level counterparts. Test your understanding of how code is translated and executed by computers!