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When the CPU excutes an instruction, the CPU may need to
fetch the input operands for
that instruction.
- The input values used by the instructions are always stored in a (general purpose) register (= very fast memory cells) that are located inside the CPU
- If the CPU needs to store the result, the CPU will always store it in a (general purpose) register that is located inside the CPU
- Most common usage of
general purpose registers:
- To store the input operands for computer instructions and
- To store the result of computer instructions
Slideshow:
(Registers provide values (= operands) for computer instructions - e.g.: addition)
(Values (= operands) are transfered to the inputs of the ALU and ALU is instructed to ADD them up)
(The result of the computation is stored in a register)❮ ❯
(Important result are transfered to memory for LONG TERM storage and safe keeping (limited number of registers !))
Example: Suppose the we want the computer to compute x + y, then:
- We (= the programmer)
must bring the
values in variables x and
y
into
some general purpose registers
For example: R0 = x and R1 = y
- We can command the
CPU to
perform an addition using
the values in the registers R0 and R1
as input
by putting the
"ADD R2,R0,R1" instruction
in the Instruction Register:
- The
"ADD R2,R0,R1" instruction
also instruct the
register R2 to
store the
result of the computation:
Note: register R2 may contain an old value. The value stored will overwrite the old value in R2
For example: when the CPU performs an add operation, the CPU needs the input values for the addition operation.
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After executing an operation, the CPU usually needs to store the result away for safe-keeping:
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- Important fact:
- A CPU only have
a limited number
(about 10 or so) of
general purpose registers
You should only use the general purpose registers to store values that will be used shortly (= soon) in some computation !!!
Assembler programming law:
- After a result is
stored in a
(general purpose) register and you don't plan to
use it for a long time, you
must store (save) the result
in the (slower) memory
for long term safe-keeping
if you need it later
Example: if the value in register R2 will not be used for some time and this value is used later, you must store the value in memory to free up the register
You will learn later in the course how to use the STR (store register) instruction to save the value from a register into memory
I depicted in the above figure what happens when the computer executes the STR instruction (the value in register R2 will be copied to some memory cell(s) -- you must specify the memory address (where to store the value) in the STR instruction. -- we will learn this later !
- A CPU only have
a limited number
(about 10 or so) of
general purpose registers
-
Example:
- If the program computes
4 + 5 + 1 + 2, the computer will
compute this sum in a number of steps:
1. Compute running sum = 4 + 5 = 9 2. Then add 1 to the running sum to obtain 10 3. Then add 2 to the running sum to obtain the final sum 12
In such a scenario, you should use:
One register (e.g. R0) to store the next number that you need to add One register (e.g. R1) to store the running sum because its used repeatedly
After you finish computing the complex expression (4 + 5 + 1 + 2), the registers R0 and R1 are "recycled" to compute the next expression
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