Language of the Computers

Hello. Here we go again. This time we’re going to discuss about no other but the language of the computer. Everybody does know that computers do not understand the daily English language human use. The users communicate with the computers by using specific machine language. Nah, now that’s the exact topic that we’re going to be discussing about here.

Now we’re introducing the Instruction Set Architecture or globally known as ISA. This ISA thing is seldom officially defining a ‘family’ of microprocessors that builds up a set of instruction to be used to communicate to a machine. The examples of ISA that are widely used are Intel x86 (IA32), Sun Spare, DEC Alpha, IBM/360, IBM PowerPC, M68K and DEC VAX. Formally and normally, the interface between a user and a microprocessor is defined by this instruction set. Internally, ISA includes instruction set, instruction encoding and some important rules of instruction using that are mnemonics, functionally and also the addressing modes.

Next is the introduction to the SPIM and MIPS. Generally, SPIM is the backward spelling of MIPS. SPIM is the simulator of MIPS that reads assembly language files of MIPS and then translates it to machine language. SPIM also executes the translated the machine language instructions before showing the contents of registers and memory. SPIM also works as the debugger that is able to support break-points and single-stepping. Other than that, SPIM provides basic operating system such as services and simple input/output. Besides, please be glad to use SPIM simulator because it is available for free online.

Right then after that, we’re discussing about the arithmetic operations in instruction language which includes addition, subtraction and many other. There will be three operands needed to operate the addition and subtraction where two operands acts as the sources whereas another one is the destination. For the code of [a,b,c], explaining a is the destination keeping the result of b plus c. As simple as that!

Now we move on to register operands. Arithmetic instructions of computer surely use register operands. For your information, MIPS has 32x32-bit of register files that are used for accessed data which numbered from 0 to 31. A 32-bit data is called a ‘word’. For temporary values, the assembler is named as $t0, $t1, $t2, $t3, $t4, $t5, $t6, $t7, $t8 or $t9. On the other hand, the assembler is named as $s0, $s1, $s2, $s3, $s4, $s5, $s6 or either $s7 if it’s a saved variable. Take note that the second Design Principle says that a smaller number produces a faster output. If a simple one states [add a,b,c ] like the previous example I gave explains ‘a’ stores the value of  ‘b’ + ‘c’, then [add $s0, $s1, $s2] means that $s0 takes the value of ‘$s1’ adds ‘$s2’.

To compare, registers are a lot faster than to be accessed than the memory. Os why use memory?

Next, let’s move on to the memory operands. Composite data like arrays, structures and dynamic data uses main memory. It is used for applying operations of arithmetic such as values loading from memory into registers and also result storing from register to the memory. In addition, memory is addressed in byte where each address identifies an 8-bit byte. Don’t forget that the addresses must be in a multiple of 4 single addresses to form a word aligned in memory.

The next one is about the representing instructions. Instructions are encoded in binary which is called as machine code or computer code. MIPS instructions are encoded as 32-bit instruction words. There are 3 instructions formats altogether, composing R-Format, I-Format and J-Format.

These are some simple formula and explanations over the all three instruction formats. The first one is the r-format instruction. Remember that when all operands in the MIPS code are registers, then the code must be not another but the r-format. The example of MIPS code using r-format instruction is as below.

Add $8, $9, $10

The second one is the j-format instruction. A code that uses this type of instruction format is the code that has one and only operand. For example is as below. As you see, the code contains one operand only.

J 10000

Last but not least is the i-format instruction. The rule of this type of format is the other than stated for r-format and j-format instructions.

bne $8, $9, -100

lw $8, 100 ($9)

addi $8, $9, 100

The next essential thing you need to know about language of the computers is addressing. The first type is the branch addressing where the instructions specify opcode, two registers and target address. Most of the branch targetsa re near the branch itself, either from the front or backward. If the branch target is too far ffset, the assembler will rewrite the code. For example,

beq $s0, $s1, L1     ->       bne $s0, $s1, L2

                        jL1

Next one is the jump addressing. Jump including ‘j’ and ‘jal’ targets can be anywhere to find in the text segment. To summarize, the addressing mode of computer instructions include all 5 mode altogether. There are register addressing, pseudodirect addressing, base addressing, immediate addressing and PC-relative addressing.

The code is said to be using register addressing mode when the code has the sources and destinations are all registers. The example is as follows.

add $8, $9, $10

Next, a machine code having one and only operand will be directly categorized as the pseudodirect addressing mode. Example:

j 10000

After that, let’s move on to the base addressing mode. A code using base addressing mode has its final operand having brackets on it. Example:

lw $8, 100 ($9)

The fourth one is the immediate addressing mode. If the final operand in the code is a constant, then the code is using this kind of addressing mode. Example:

addi $8,$9, 100

Finally, let’s talk about PC-relative addressing mode. The code’s output is a branch. For this mode, the code usually includes the operation of Branch on Equal (beq), Branch on Not Equal (bne) and so on that has the initial b. The example is as follow:

bne $8, $9, -100

In conclusion, we have three instruction formats and five addressing modes to be understood of. They are all as simple as a b c. You just need to put your interest and confide in yourself to improve. Okay, see you later in the next entry!

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