RISC stands for “Reduced Instruction Set Computer”. It is one of the paradigms used to design an ISA (Instruction Set Architecture). It implements very simple instructions having a simple and regular structure. The number of instructions is also less about 64-128.
Examples: IBM, ARM, etc.
Assembly Language is a low-level programming language that is specific to a particular ISA and uses very simple statements that correspond to one machine instruction.
It is a highly efficient programming language for writing codes for operating systems and device drivers.
Program stands for a “Computer Program” which is a set of statements written by the programmers following the correct syntax to perform a certain task on computers.
Also Read Types of Looping statements in C/C++
Simple RISC Assembly Instructions
Any simple RISC assembly statement has two main parts:
- Instruction also called “Opcode” stands for Operation Code. It has some special meaning to the machine.
- Operands these can be a resister, some memory locations, or a constant also called an “immediate”.
In simple RISC there are a total of 21 assembly instructions, but here we will see the very basics ones.
- mov instruction is used to transfer the contents of a resister or an immediate to another resister. It is the most commonly used instruction while writing simple RISC assembly programs.
Example: mov r0, 5 or mov r0, r1
There are in total 6 arithmetic instructions in simple RISC
- add instruction is used to add the contents of a resistor and an immediate or the contents of two resistors and assigns the result to a new resister.
Example: add r1, r0, 9 or add r2, r1, r0 (r1 = r0 + 9 or r2 = r1 + r0)
- sub instruction is used to subtract the contents of a resister or an immediate from another resister and assigns the result to a new resister.
Example: sub r1, r0, 7 or sub r2, r1, r0 (r1 = r0 – 7 or r2 = r1 – r0)
- mul instruction is used to multiply the contents of a resister and an immediate or the contents of two resistors and assigns the product to a new resister.
Example: mul r1, r0, 5 or mul r2, r1, r0 (r1 = r0 x 5 or r2 = r1 x r0)
- div instruction is used to divide the contents of a resister by the contents of an immediate or a resister and assigns it to a new resister.
Example: div r1, r0, 6 or div r2, r1, r0 (r1 = r0 / 6 or r2 = r1 / r0)
- mod instruction is used to divide the contents of a resister by the contents of an immediate or a resister and assigns the remainder to a new resister.
Example: mod r1, r0, 3 or mod r2, r1, r0
- cmp instruction is a compare instruction used to compare the contents of two resisters. It makes use of a special resister called flags.
Example: cmp r0, r1 (r0 > r1 or r0 < r1 or r1 == r0)
The value of flags.E = 0 & flags.GT = 1 when the contents of the first resister are greater than the second one, flags.E = 1 & flags.GT = 0 when the contents of the first and the second resisters are equal, and flags.E = 0 & flags.GT = 0 when the contents of the first is less than the second one.
NOTE: flags resisters stores the value of the last comparison.
Conditional Branch Statements
- beq .label stands for (branch if equal) branch to .label i.e. transfer the control of execution to the indicated label when flags.E = 1
- bgt .label stands for (branch if greater than) branch to .label i.e. transfer the control of execution to the indicated label when flags.GT = 1
NOTE: The values of flags resisters are only set by cmp instruction so to use the conditional branch statements we must use cmp instruction before these.
Let’s write a simple RISC assembly program to calculate the factorial of a number
@ This is a factorial program
mov r0, 1 /* r0 will store the final factorial result */
mov r2, r1 /* r1 is the number whose factorial has to be calculated */
.loop: /* starting of the loop */
mul r0, r0, r2 /* r0 = r0 x r2 */
sub r2, r2, 1 /* r2 = r2 - 1 */
cmp r2, 1
bgt .loop
The final value of r0 will give the required result i.e. factorial of a number (r1)Output:
r1 = 5
r0 = 120