The essence of a computer can be captured in some very simple properties:
A simple view of MU0 considers it as having just 2 components and a link
The memory is a set of labelled (i.e. numbered) locations which can hold information, such as numbers. The labels are known as addresses.
The bus is a bidirectional communications path, transmitting both addresses and numbers. (In practice, there may be several buses.)
The processing unit (usually referred to as the central processing unit, or CPU) can:
What the CPU actually does is controlled by a set of sequentially executed instructions, known as a program.
e.g. suppose that our example computer can perform the following actions:
|LDA||x||copies the contents of location x to the ACC|
|ADD||y||adds a copy of the contents of location y to the ACC|
|STO||z||copies the contents of the ACC to location z|
|STP||stops the computer|
These instructions are enough to add some numbers together and save the total e.g. we could use it in a cash till, to add the cost of all the individual items together to give the total cost:
First we have to decide on the representation of prices inside the computer. A simple story is that #1.09 is represented by 109 and #15.34 by 1534. (We are ignoring problems such as the number base, the maximum number we can represent, and negative numbers - see CS1031.)
Now, suppose we somehow have the running total held in location 10 and the
item price in location 11 (how this might be achieved is explained in CS1031):
we could use the program:
If we want to process more than one item, we somehow have to repeat this program, so we need to explore how the sequence of instructions can be controlled:
The list of instructions is clearly a vital part of the computational process and following it automatically is an essential part of the operation of a computer.
All practical digital computers use their memory to hold instructions as well as data (i.e. numbers or any other information that the program is working on) - thus "stored program". Computers have a Program Counter (PC) in the CPU which contains the address of the memory location containing the next instruction to be obeyed (executed). (Intel calls this the Instruction Pointer or IP.)
To be stored in memory locations, instructions have to be represented by numbers. The way in which an operation (encoded as a small integer e.g. LDA=0, STO=1, ADD=2) and a location are represented by a single number is similar to the way in which both pounds and pence are squeezed into a single number in the example above. The details of this representation are explained in the lab script, and can be seen using the simulator.
At the start of the program described above, the picture is now something
The computation starts with the PC set pointing to the address of the first instruction, in this case 0 (an arbitrary choice, as used in the MU0 simulator). The computation proceeds by executing the instruction pointed to by the PC and incrementing the PC by one. When the STP instruction is reached this process is halted.
With linear lists of instructions, we can do limited things. Most of the power of computers comes from the power to change between different sequences of instructions as a result of tests on the stored information.
JMP, JNE & JGE, known collectively as jump instructions, allow us to
change from one sequence of instructions to another. The second part of each
jump instruction is the address of the start of the next sequence of
JMP is known as an unconditional jump, as it always causes a change, whereas JNE and JGE are known as conditional jumps, as they may or may not cause a change, depending on the current value in ACC: JNE causes a change only if the value is Not Equal to zero, and JGE if the value is Greater than or Equal to zero.
Suppose, in our cash till example, each new item price was magically put
into memory location 11, and this was to be added to the running total until
the item price was zero:
|JNE||more||if item non-zero then|
|more:||ADD||10||add non-zero item to total|
|JMP||again||and try again|