SEP105
Dr. Benjamin Champion
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What is a Program?
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What is a program?
You should have an appreciation of what a computer program is from modern day to day life.
Essentially a program is a set of instructions that is executed by a Central Processing Unit
(CPU) to complete a task or process.
So is it that simple? From a high level, yes. But how does this actually work?
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What is a program?
When we are writing software, regardless of the computer language that we are using, the
program must be converted into machine code before it can be executed. This might be done
at compile time of the program, or as the interpreter interprets the program.
Machine code is the lowest possible form of code. Machine code is the 1’s and 0’s that make
the computer work! While it is possible to write programs in machine code, you would almost
never do this on a modern device as it is very slow to write, device specific, easy to make
mistakes and can be difficult to debug
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Assembly Code
In its simplest state, machine code
consists of numbers that our CPU can
interpret as instructions and addresses.
In its human readable format, this is
called assembly code. These instructions
are typically for relatively simple tasks,
like load a value from memory, add two
values together, etc.
Because of this, a PC cannot think! It can
simply follow the instructions that it is
given. These instructions can be
combined together and executed very
quickly to give a PC the illusion of
intelligence but in reality, it is confined to
the instructions that it is able to read from
the predefined machine code.
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Instruction Function
add
Add two values together
sub
Subtract two one value from another
ld
Load data
st
Store data
Push
Push a value onto the stack
Pop
Pop (retrieve) a value from the stack
jmp
Jump to a different address (change the value of the PC)
etc
Fetch, Decode, Execute cycle
Once the machine code has been developed, how is this code actioned on by the CPU? In
most modern CPUs, the Fetch-Execute cycle is used (or a derivative there of).
The Fetch Execute cycle can be described using a simplified model with only the following
components:
• Program Counter – This is a value stored in a register that knows the current memory
address of the program. An instruction is loaded from this address. For the program to
continue, the Program Counter must change (eg be incremented) or the same value will
always be executed.
• Instruction Register – This is the current instruction that is being worked on by the CPU
• Accumulator Register – This is a temporary memory location that can have work done to
it (eg store a value to add something to it)
• Memory – This is somewhere where the program and values are being stored, like RAM
• Clock – This is a signal that drives how fast the cycle can execute. Everything in a PC
happens on a clock cycle, not in-between! Therefore, the faster the clock, the faster the PC
will process commands.
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Fetch, Decode, Execute cycle
The system might look something like this
CPU
MEMORY
Program Counter (PC)
Instructions
Instruction Register (IR)
Accumulator (Ac)
Clock (CLK)
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Storage
Fetch
First the instruction is FETCHed from memory. The program counter is used to determine which
instruction we should retrieve from memory. Initially, the program counter is set to 0. Therefore, on
the first clock cycle the first instruction is loaded from memory into the instruction register.
MEMORY
CPU
PC = 0x00
Instructions
IR = ld 5
Ac
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
23
0x06
7
0x07
45
Decode
Next the instruction is DECODEd. Typically, an instruction will have what it wants to do, and where it
wants to do it. On the second clock cycle, the CPU will get ready to execute the instruction from the
specific memory location
MEMORY
CPU
PC = 0x00
Instructions
IR = ld 5
Ac
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
23
0x06
7
0x07
45
Execute
Next the instruction is EXECUTEd. At this point, the accumulator can be used to store the result. This
may take several clock cycles, depending on the instruction that is being executed.
MEMORY
CPU
PC = 0x00
Instructions
IR = ld 5
Ac = 23
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
23
0x06
7
0x07
45
Fetch-Execute Cycle
Finally, the program counter is incremented, and the Fetch command is repeated again. This will load
in the next instruction from memory, and the process repeats until we run out of instructions to
execute!
MEMORY
CPU
PC = 0x01
Instructions
IR = add 6
Ac = 23
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
23
0x06
7
0x07
45
Jump
When we get to the end of the program, often we may want to go back to the start of the program. Or
at some point in time we may want to run a different part of the program (eg function). To do this, we
can use the jump command!
Fetch
MEMORY
CPU
PC = 0x03
Instructions
IR = jmp 0x00
Ac = 30
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
30
0x06
7
0x07
45
Jump
When we get to the end of the program, often we may want to go back to the start of the program. Or
at some point in time we may want to run a different part of the program (eg function). To do this, we
can use the jump command!
Decode
MEMORY
CPU
PC = 0x03
Instructions
IR = jmp 0x00
Ac = 30
CLK
Deakin University CRICOS Provider Code: 00113B
Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
30
0x06
7
0x07
45
Jump
When we get to the end of the program, often we may want to go back to the start of the program. Or
at some point in time we may want to run a different part of the program (eg function). To do this, we
can use the jump command!
Execute
MEMORY
CPU
PC = 0x00
Instructions
IR = jmp 0x00
Ac = 30
CLK
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Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
30
0x06
7
0x07
45
Fetch-Execute Cycle
Then the program can loop
MEMORY
CPU
PC = 0x00
Instructions
IR = ld 5
Ac = 30
CLK
Storage
Address
Instruction
0x00
ld 5
0x01
add 6
0x02
st 5
0x03
jmp 0x00
0x04
4
0x05
30
0x06
7
0x07
45
Good explanation: https://www.youtube.com/watch?v=Z5JC9Ve1sfI&ab_channel=TomScott
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