The most interesting part of computer engineering is the programming! Specially when you know how it works, you can make better programs. Now, we designed one, and we want to program it. But wait, let's take a look at computers made before.
As you may remember from previous chapters, we had this for showing a simple character on console:
STK SEGMENT
DW 32 DUP(?)
STK ENDS
CDS SEGMENT
ASSUME CS:CDS, SS:STK
MAIN PROC FAR
MOV DL, 'A'
MOV AH, 02
INT 21h
MOV AH, 4Ch
INT 21h
MAIN ENDP
CDS ENDS
END MAINAnd in chapter fourteen, we learned that this code will become Machine Code. And now, we can assemble our own code, and write programs for that.
Assembler is actually a software, used to generate machine code. Days we had no assembler, we had to generate machine code ourselves. This is hard, isn't it? But that's the only way when you have no operating system or other computer to assemble your code. Actually, a computer uses Assembler , then Linker to understand the programs. But in case of simple computers like ours, a simple assembler is enough. We just want to fill RAM blocks. The assembler sees our code. For example :
AND 10, 12
and then, as we have two 8 bit inputs, it converts the 10 to 000010101 and 12 to 00001100. And it will look at this table :
| Ainvert | Bnegate | S2 | S1 | Instruction |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | AND |
| 0 | 0 | 0 | 1 | OR |
| 1 | 1 | 0 | 0 | NOR |
| 1 | 1 | 0 | 1 | NAND |
| 0 | 0 | 1 | 0 | ADD |
| 0 | 1 | 1 | 0 | SUB |
and finds that code for AND is 0000. Then generates this 0x00A0C as the final code! Let's write long numbers in hexadecimal.
When we use hex, a 20-bit number will be a 5 digits number. So, we are the assembler in this case. Let's write our codes :
AND 10, 12
# Hex : 0x00A0C
ADD 3, 8
# Hex : 0x20308
SUB 10, 8
# Hex : 0x60A08
And this is a simple program in the machine language :
0000 00001010 00001100
0010 00000011 00001000
0110 00001010 00001000
Yes, computer only understands one and zero (please re-study chapter two, if you forgot this.) and we need to generate a bunch of ones and zeros to make a computer work! But, computers actually need a good interface between the system and the user. What's that?
Operating System, is the interface! It connects the system to the user. Operating System is a software, which lets other software be installed and ran on your computer. If you have a PC in home, it may run Windows or Linux as the operating system. If you have a Mac, it may run macOS. And if you have an iPhone, it runs iOS. There are a lot of operating systems in the world, but a few operating systems are usable. Why? because other ones are only made for a special purpose. For example, MINIX is made for educational purposes. If you want to learn how UNIX works, you can study MINIX. But operating systems such as Linux, Windows or macOS are general-purpose. They're made to be the interface.And now you may ask , will we write an operating system for our computer? Sorry, No! If you pay attention to our assembly code, you will find this line :
MOV AH, 4Ch
INT 21h
The part including INT is actually an interrupt. We have no interrupts in our computer. This is the first problem. We also have no loops in our
computer, in instruction set, we didn't define anything to make a loop and any recursive structure. 8086 and many other real processors, usually
have instructions like branch or jump, and those instructions help us make real software, including operating systems! So, we never write operating
system for our little computer.
Now, you know a lot about computer architecture and organization. Also, you can make and program a computer in gate-level. This is necessary, trust me. You always need to know how computers work, otherwise you won't be able to make software. But I'm sure you're curious about real hardware design. The next chapter (and the last one!) is about that. You will learn how companies design and produce their own hardware.