Machine Code

As usual, the bit is specified by giving the number of its position, followed by either a variable or expression. If the value is an expression, it is assumed to be an address, and great care should be taken.

BCLR
instruction: clear a bit
Bclr number,value

The BCLR command clears a bit by setting it to zero. The bit number can be from 0 to 31, and pinpoints the single binary digit to be cleared. If the value is an expression, it will be used as an address in memory.

Using assembly language

AMOS Professional exploits the most useful machine code routines and transforms them into simple Basic commands. Even if you are an experienced machine code programmer, you will have to work very hard to better the speed and range of AMOS Professional.

Even though assembly language is hazardous and you are best advised to avoid it, there are a few routines which could be improved by its use. So for this reason, you are provided with several methods of accessing machine code directly from AMOS Professional Basic. These features are strictly for experts, and should be ignored by anyone not familiar with assembly language.

Machine code procedures
The easiest option for exploiting assembly language is to install machine code directly into an AMOS Professional procedure. These procedures can be saved and loaded using standard commands, and then executed from the Basic program simply by typing their name! Apart from the fact that it cannot be listed on screen, the only effective difference between a machine code procedure and its Basic equivalent is speed.

Machine code procedures are completely compatible with the AMOS Compiler, which will not only run most of your programs at double speed, but also compact them to a fraction of their original size. This means that if you decide to compile your programs at a later date, you will not need to alter your assembly language at all. The following points should be noted before using an assembler:

• Routines should be re-locatable, able to run under CLI and should end with a simple RTS instruction.
• The only limit to the size of the machine code is the amount of available memory. However, only the first CODE segment should be used for programs. The contents of any other segment will be completely ignored!
• If memory is reserved using the Amiga system functions, remember to return reserved memory to the memory pool after use. AMOS Professional cannot be expected to know or care what you are doing!
• A procedure will be moved in memory every time a line is entered via the Editor. This means that if an interrupt is attached to the routine, it must be removed before anything in the Basic program is changed, otherwise the Amiga will crash!