collapseos/parts/README.md

Parts

Bits and pieces of code that you can assemble to build an OS for your machine.

These parts are made to be glued together in a single main.asm file you write yourself.

As of now, the z80 assembler code is written to be assembled with scas, but this is going to change in the future as a new hosted assembler is written.

Defines

Each part can have its own constants, but some constant are made to be defined externally. We already have some of those external definitions in platform includes, but we can have more defines than this.

Each part has a “DEFINES” section listing the constant it expects to be defined. Make sure that you have these constants defined before you include the file.

Variable management

Each part can define variables. These variables are defined as addresses in RAM. We know where RAM start from the RAMSTART constant in platform includes, but because those parts are made to be glued together in no pre-defined order, we need a system to align variables from different modules in RAM.

This is why each part that has variable expect a <PARTNAME>_RAMSTART constant to be defined and, in turn, defines a <PARTNAME>_RAMEND constant to carry to the following part.

Thus, code that glue parts together coould look like:

MOD1_RAMSTART .equ RAMSTART
#include "mod1.asm"
MOD2_RAMSTART .equ MOD1_RAMEND
#include "mod2.asm"

Code style

The asm code used in these parts is heavily dependent on what scas offers. I try to be as “low-tech” as possible because the implementation of the assembler to be implemented for the z80 will likely be more limited. For example, I try to avoid macros.

One exception, however, is for the routine hooks (SHELL_GETC for example). At first, I wanted to assign a label to a const (SHELL_GETC .equ aciaGetC for example), but it turns out that scas doesn't support this (but it could: label addresses are known at compile time and thus can be consts (maybe at the cost of an extra pass though)). I went for macros instead, but that doesn't mean that the z80 assembler will need to support macros. It just need to support labels-as-consts.