1. Assembly Language Assembly Language
A Level Only
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2. Machine Code
Each type of CPU is designed to carry out a set of specific instructions. Each instruction is represented by a binary number. This is called machine code.
Machine Code
Assembly Language
MOV R3, R9
SUB R1, R3, #10
ADD R0, R0, R1
It is hard for humans to read and write machine code so assembly language was developed to make it easier.
Assembly language features a set of mnemonics, each one representing one machine code instruction.
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3. These are the mnemonics that are commonly used in assembly language:
Description
LDR
Load from memory location into a register
ADD
Add data in a register
SUB
Subtract data in a register
STR
Store values from register into a memory location B
Branch to a marked position in the program
CMP
Compare values in a register
AND
Bitwise logical AND operation
ORR
Bitwise logical OR operation
XOR
Bitwise logical XOR operation
MVN
Bitwise logical NOT operation
LSL
Logically shift the value left, used for multiplication
LSR
Logically shift the value right, used for division
HALT
Stops the program
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4. Instruction Format ADD MOV SUB R0, R1, #5 R0, R2 R2, R2, R4
This is the standard format for writing instructions in assembly language.
opcode: the operation code is a single instruction
operands: the values the instruction will use
ADD
R0, R1, #5
MOV
R0, R2
SUB
R2, R2, R4
operation
destination
input
input
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5. Storage
Two different types of storage are used when the CPU is executing instructions.
Registers
Storage locations contained within the CPU which are much faster to access than memory
Memory
The RAM is located outside the CPU and is slower to access than registers.
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6. Addressing
There are different methods to access data and instructions in memory; these are called memory addressing modes.
Immediate
The data is hard-coded into the instruction. It is the fastest method as it doesn’t use memory.
Direct
The instruction refers directly to a location in memory (either a register or a memory location).
ADD
R0, R1, #5
Direct
Direct
Immediate
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7. Example LDR LDR SUB STR R0, 102 R1, 101 R2, R0, R1 R2, 100 Registers
Loads the value from memory location 102 into register 0
LDR
R0, 102
Registers R0 20 R1 5 R2 15
Loads the value from memory location 101 into register 1
LDR
R1, 101
Subtracts the value in R1 from the value in R0, stores the result in R2
SUB
R2, R0, R1
Memory
100 15
101 5
102 20
Stores the value from R2 in memory location 100
STR
R2, 100
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8. Logical Shift 1 1 1 = 45 = 90 (×2) = 180 (×4)
Multiplication and division are very CPU-intensive operations; logical shift is an alternative method that can be used for multiplication and division by powers of 2. 1 1
= 45 1
= 90 (×2)
= 180 (×4)
As you can see, shifting the number one place to the left is the equivalent to multiplying by 2 and shifting by two places is the equivalent to multiplying by 4.
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9. Branching is used to jump to different points in the program.
Opcode
Operands
Description
MOV
R0, #50
The value 50 is moved into register 0
R1, #0
The value 0 is moved into register 1
CMP
R0, R1
The values stored in registers 0 and 1 are compared
BGT
numIsGT
If the value stored in register 0 is greater than the value stored in register 1, the program branches to the numIsGT label
STR
R1, 100
HALT
numIsGT:
A label used to define a point in the program
R0, 100
Stores the value stored in register 0 in memory location 100
Stops the program
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10. END