1. Lesson Objectives Aims Understand how machine code is generated
Understand the role of Linkers and Loaders during compilation

2. Machine Code Generation
AST
Identify Machine Code Blocks
Binary Executable

3. The compiler will perform the following actions:
Convert AST nodes in to machine code
Attempt a best fit model of allocating registers
Machine code optimisation

4. Converting to machine code
Once code is split in to atomic elements, it becomes much easier to convert to machine code
Languages all use pre-defined actions such as :
Assignment
Comparison
Branching
These can all be mapped to pre existing machine code templates

5. Allocation of registers
During any operation, registers will be used
CPU’s only contain a certain number of registers
Usually, each variable will need access to a register in order to be manipulated
This means the compiler must assign registers to variables and some will share registers – clashes must be avoided!

6. This is a non deterministic problem
In other words, the “correct” answer can either never be found or is so computationally expensive you’d die before it did.
Just like a sat-nav…

7. What kills CPU efficiency?
Optimisation
What kills CPU efficiency?
The optimisation stage will effectively try to rework code so that cpu disruption is minimised:
Removing unnecessary branches
Predicting probable program flow
Using any CPU extensions/specific instructions instead of “generic” code (targeted compilation)

8. Linking and loading
Libraries are a god send
Back in t’ day, you had to code EVERYTHING yourself
Then there became a need for “middleware” and libraries
Why?

9. At this point, the compiler has to make sure that the libraries are included in the final executable.
These can be:
Static
dynamic

10. Linker/Loader
The linker will tell your code how to access external resources
Static linking is where the library is COPIED to the executable
This has obvious advantages and disadvantages…

11. Dynamically linked libraries are just that – linked
They keep file size small
Can be updated without affecting the executable
May not be present!

12. Loaders
Loaders handle the connections to a dynamic library – loading them in to memory when necessary
They are small utilities that handle the interaction between program and library
This is because we don’t always know where in memory a dynamic library will reside

13. Review/Success Criteria
You should know:
How code goes from high level languages to an executable!