Subtopic Notes
4.2 Assembly Language
4. Processor Fundamentals
Machine code
Binary written code that uses the processor’s basic machine operations
Assembly language:
- Low-level programming language
- Instructions made of an opcode and operand
- Each assembly language instruction (source code) translates into exactly one machine code instruction (object code)
- Symbolic Addressing
- Mnemonics denotes operation codes e.g. LDM, STO
- Labels may be used for addresses
- Absolute addressing: A fixed address in memory
Assembler
- Software that translates low-level assembly language into machine code
- Substitutes all labels and mnemonics with their corresponding binary values
- The binary values are predefined by the assembler
- One Pass Assembler
- Translates the mnemonic source code to machine code in a single pass
- Unable to process code that includes forward referencing
- Two Pass Assembler
- First Pass
- Symbol table is made to map symbolic addresses and labels to specific memory locations
- Errors are flagged when detected
- Second Pass
- Symbolic addresses are replaced by absolute addresses
- Jumps and forward referencing instructions are resolved
- Complete source code translated to machine code
- Errors are reported if exists
- Object code generated
- First Pass
Assembly Language Instructions
| Opcode | Operand | Explanation |
|---|---|---|
| Modes of Addressing | ||
| LDM | #n | Immediate addressing. Load the number n to ACC |
| LDD | <address> | Direct addressing. Load the contents of the location at the given address to ACC |
| LDI | <address> | Indirect addressing. The address to be used is at the given address. Load the contents of this second address to ACC. |
| LDX | <address> | Indexed addressing. Form the address from <address> + the contents of the index register. Copy the contents of this calculated address to ACC |
| LDR | #n | Immediate addressing. Load the number n to IX |
| Data movement | ||
| MOV | <register> | Move the contents of the accumulator to the given register (IX) |
| STO | <address> | Store the contents of ACC at the given address |
| Arithmetic operations | ||
| ADD | <address> | Add the contents of the given address to the ACC |
| ADD | #n/Bn/&n | Add the number n to the ACC |
| SUB | <address> | Subtract the contents of the given address from the ACC |
| SUB | #n/Bn/&n | Subtract the number n from the ACC |
| INC | <register> | Add 1 to the contents of the register (ACC or IX) |
| DEC | <register> | Subtract 1 from the contents of the register (ACC or IX) |
| Unconditional and conditional instructions (jumps) | ||
| JMP | <address> | Jump to the given address |
| JPE | <address> | Following a compare instruction, jump to <address> if the compare was True |
| JPN | <address> | Following a compare instruction, jump to <address> if the compare was False |
| Compare instructions | ||
| CMP | <address> | Compare the contents of ACC with the contents of <address> |
| CMP | #n | Compare the contents of ACC with number n |
| CMI | <address> | Indirect addressing. The address to be used is at the given address. Compare the contents of ACC with the contents of this second address |
| Input and output of data | ||
| IN | Key in a character and store its ASCII value in ACC | |
| OUT | Output to the screen the character whose ASCII value is stored in ACC | |
| Terminate | ||
| END | Return control to the operating system |
All questions will assume there is only one general purpose register available
(Accumulator)
ACC denotes Accumulator
IX denotes Index Register
<address> can be an absolute or symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B0101010
& denotes a hexadecimal number, e.g. &4A
| Label | Opcode | Operand | Explanation |
|---|---|---|---|
| <label>: | <opcode> | <operand> | Labels an instruction |
| <label>: | <data> | Gives a symbolic address <label> to the memory location with contents <data> |