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54 lines
2.1 KiB
Markdown
54 lines
2.1 KiB
Markdown
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# Z80 Parts
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Bits and pieces of code that you can assemble to build an OS for your machine.
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These parts are made to be glued together in a single `main.asm` file you write
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yourself.
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As of now, the z80 assembler code is written to be assembled with [scas][scas],
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but this is going to change in the future as a new hosted assembler is written.
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## Defines
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Each part can have its own constants, but some constant are made to be defined
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externally. We already have some of those external definitions in platform
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includes, but we can have more defines than this.
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Each part has a "DEFINES" section listing the constant it expects to be defined.
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Make sure that you have these constants defined before you include the file.
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## Variable management
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Each part can define variables. These variables are defined as addresses in
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RAM. We know where RAM start from the `RAMSTART` constant in platform includes,
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but because those parts are made to be glued together in no pre-defined order,
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we need a system to align variables from different modules in RAM.
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This is why each part that has variable expect a `<PARTNAME>_RAMSTART`
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constant to be defined and, in turn, defines a `<PARTNAME>_RAMEND` constant to
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carry to the following part.
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Thus, code that glue parts together coould look like:
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MOD1_RAMSTART .equ RAMSTART
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#include "mod1.asm"
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MOD2_RAMSTART .equ MOD1_RAMEND
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#include "mod2.asm"
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## Code style
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The asm code used in these parts is heavily dependent on what scas offers. I
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try to be as "low-tech" as possible because the implementation of the assembler
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to be implemented for the z80 will likely be more limited. For example, I try
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to avoid macros.
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One exception, however, is for the routine hooks (`SHELL_GETC` for example). At
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first, I wanted to assign a label to a const (`SHELL_GETC .equ aciaGetC` for
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example), but it turns out that scas doesn't support this (but it could: label
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addresses are known at compile time and thus can be consts (maybe at the cost
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of an extra pass though)). I went for macros instead, but that doesn't mean
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that the z80 assembler will need to support macros. It just need to support
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labels-as-consts.
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[scas]: https://github.com/KnightOS/scas
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