| Programming with Ophis | ||
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A macro is a way of expressing a lot of code or data with a simple shorthand. It's also usually configurable. Traditional macro systems such as C's #define mechanic use textual replacement: a macro is expanded before any evaluation or even parsing occurs.
In contrast, Ophis's macro system uses a call by value approach where the arguments to macros are evaluated to bytes or words before being inserted into the macro body. This produces effects much closer to those of a traditional function call. A more detailed discussion of the tradeoffs may be found in the Appendix called Ophis Command Reference.
A macro definition is a set of statements between a .macro statement and a .macend statement. The .macro statement also names the macro being defined.
No global or anonymous labels may be defined inside a macro: temporary labels only persist in the macro expansion itself. (Each macro body has its own scope. A label map will trace back through macro expansions to describe were a label inside a macro body came from.)
Arguments to macros are referred to by number: the first is _1, the second _2, and so on.
Here's a macro that encapsulates the printing routine in our "Hello World" program, with an argument being the address of the string to print:
.macro print
ldx #0
_loop: lda _1, x
beq _done
jsr chrout
inx
bne _loop
_done:
.macend |
Macros may be invoked in two ways: one that looks like a directive, and one that looks like an instruction.
The most common way to invoke a macro is to backquote the name of the macro. It is also possible to use the .invoke command. These commands look like this:
`print msg .invoke print msg |
Arguments are passed to the macro as a comma-separated list. They must all be expressions that evaluate to byte or word values—a mechanism similar to .alias is used to assign their values to the _n names.
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