Why its called as Virtual Machine?

Datetime:2016-08-22 22:51:33          Topic: Assembler  Coder           Share

Soon developers started denoting the numbers in hex where the above machine language can be written as

CC 0A,03

There are still some universities having 8085 based programming paper which gives this mode of entering programs in to computer. Even today we can open executable files and see it in hex format.

Everything in computer is actually in binary but for better understanding purpose, we can convert it to hex and display. Don't get confused with the ASCII representation at the right side. That is just coincidence. For example the opcode for add and ASCII code for 'D' may be same. Here the viewer application from where the screen shot is taken is trying to convert to ASCII for more readability.

Assmembly language

Soon people started using more human friendly instructions instead of hex instructions. That was called as assembly language. The above code can be written in assembly language as follows

Add 0A,03

Only the instruction / opcode changed data remained in hex.

Smart people created text editors in assembly language to write assembly language. That made the life of new developers easy as they can write programs editor which is built using assembly language itself.

There are no loops or most of the machines doesn't supported it. But we had an option to change the execution order based on conditions. Instructions such as JMP, JC JNC (Jump, Jump on Carry, Jump on No Carry) are some examples. Using these instructions developers were able to get loops implemented.

Till this point the developers were directly writing in the language the machine / hardware understands. After the assembly language, there were some programs started involving in between the program and machines.

Programs that prepare programs for execution

The concepts like macros , pre-processors etc...introduced the need for particular programs to prepare other programs for execution. In other words, if we are writing assembly language using English opcodes that needs to be converted to corresponding machine code. If we consider the above sample of

Add 0A,03

Someone has to convert this text 'Add' opcode to CC which is the binary equivalent of 11001100 for add instruction. The programs which are doing this conversion is called Assembler.

Still the assmebly language has instrcutions which can be directly mapped to an instruction which machine support. ie we have to read the manual of processor chip to know what are the instructions it supports.

High level languages

One to one relation between assembly language instruction to machine instruction was some kind of limitation and later people thought of more intuitive languages to program. A more human readable approach where instructions in program doesn't needs to have one to one relation to the machine code. Like assemblers, the program can be converted to machine language before execution. When converting, one instruction in this language can be converted as one more more machine instructions.

Yes the high level languages are born. Excellent example is C. We write in high level more human readable format and converted / compiled to machine instructions before execution.

This produced the concept of loops, functions variable types etc...Earlier it was all just bits and bytes with a jump instruction.

Internally loops might be using jump instructions, but to the developer, the details of machine was hidden. This helped more and more people to become developers. The don't need to how the machine works in the chip level or what is voltage or binary but still able to write programs.

Summary - unmanaged world

We can summarize this as a model where we prepare machine instructions either writing directly, or converting / compiling assembly or high level languages. Once it is ready just start execution. There is no one to manage its the machine and it just executes our code. 





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