Just Enough Abstraction

What is really important here is that the move to a symbolic representation has once again distanced us from the machine code.

Now we have something that looks quite sophisticated and is just one step away from the idea of a “variable”, a key concept in the development of all computer languages.

You can start to forget about “addressing” and the physical hardware that makes the machine work. Instead you can start to think about creating variables – named units of storage – like PayRate that you can simply use in your program without any worry about where they are actually stored.

The elaboration of the “variable” idea gave rise to the first high-level computer language – Fortran.

Oddly assembly languages didn’t just take the totally abstract approach. Even today,  if you want to use assembler then staying close to the machine’s hardware must be important to you. As a result assembly language programmers have found ways of making their products just abstract enough to make  them easier to use but without reducing the efficiency of the program or the ability of the programmer to make things happen exactly as required.

So, as well as direct symbolic addressing, most assembly languages have a whole range of different types of addressing that can be used to access data. For example, register relative addressing, as in:

MOV AX,[BX]+10

means move the contents of the memory location who's address is given by adding 10 to the contents of the BX register, into register AX.

Notice that the square brackets generally mean – the “address given by”.  This sort of notation eventually developed into the high-level language concept of an array of variables.

For example, you might write PayRate[10] to mean the 10^th variable in the PayRate array which would translate more or less directly to [BX]+10. Oddly the concept of an array, or any sophisticated data structure, never caught on as part of an assembler – presumably because it was a step too far away from the hardware.

Assemblers slowly developed additional features and one of the most powerful is the idea of the macro. A macro is simply a block of text that you can insert into a program by specifying its name. So if you keep using the same block of assembler you can save a lot of effort by defining a macro for it and using that. Of course things can get much more sophisticated. Macros soon developed parameters so you could write reusable blocks of code that could be customized when used.

Today most assemblers are "macro" assemblers and they even come with lots of predefined macros that make assembler closer to a high level language. However the key fact is that to be an assembler it must be possible for the programmer to get back to the lowest possible level - the assembly language that is very close to the way the machine actually works.

More Information

Microsoft Macro Assembler

What Programmers Know

Contents

1. The Computer - What's The Big Idea?*
2. The Memory Principle - Computer Memory and Pigeonholes*
3. Principles of Execution - The CPU
4. Assemblers and assembly language*
5. The Essence Of Programming
6. Variables - Scope, Lifetime And More*
7. Binary Arithmetic
8. Hexadecimal*
9. Binary - Negative Numbers*
10. Floating Point Numbers*
11. Inside the Computer - Addressing
12. The Mod Function
13. Recursion
14. The Lost Art Of The Storage Mapping Function *
15. Hashing - The Greatest Idea In Programming
16. Advanced Hashing
17. XOR - The Magic Swap*
18. Programmer's Introduction to XML
19. From Data To Objects*
20. What Exactly Is A First Class Function - And Why You Should Care*
21. Stacks And Trees*
22. The LIFO Stack - A Gentle Guide*
23. Data Structures - Trees
24. Inside Random Numbers
25. The Monte Carlo Method
26. Cache Memory And The Caching Principle
27. Data Compression The Dictionary Way
28. Dates Are Difficult*
29. Sequential Storage*
30. Magic of Merging*
31. Power of Operators
32. The Heart Of A Compiler*
33. The Fundamentals of Pointers
34. Functional And Dysfunctional Programming*

* Recently revised

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