mirror of
https://github.com/hsoft/collapseos.git
synced 2024-11-08 10:18:05 +11:00
118 lines
4.6 KiB
Markdown
118 lines
4.6 KiB
Markdown
## Code conventions
|
|
|
|
The code in this project follow certain project-wide conventions, which are
|
|
described here. Kernel code and userspace code follow additional conventions
|
|
which are described in `kernel/README.md` and `apps/README.md`.
|
|
|
|
## Defines
|
|
|
|
Each unit 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.
|
|
|
|
Many units have 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 unit 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 unit that has variable expect a `<PREFIX>_RAMSTART`
|
|
constant to be defined and, in turn, defines a `<PREFIX>_RAMEND` constant to
|
|
carry to the following unit.
|
|
|
|
Thus, code that glue parts together could look like:
|
|
|
|
MOD1_RAMSTART .equ RAMSTART
|
|
#include "mod1.asm"
|
|
MOD2_RAMSTART .equ MOD1_RAMEND
|
|
#include "mod2.asm"
|
|
|
|
## Register protection
|
|
|
|
As a general rule, all routines systematically protect registers they use,
|
|
including input parameters. This allows us to stop worrying, each time we call
|
|
a routine, whether our registers are all messed up.
|
|
|
|
Some routines stray from that rule, but the fact that they destroy a particular
|
|
register is documented. An undocumented register change is considered a bug.
|
|
Clean up after yourself, you nasty routine!
|
|
|
|
Another exception to this rule are "top-level" routines, that is, routines that
|
|
aren't designed to be called from other parts of Collapse OS. Those are
|
|
generally routines close to an application's main loop.
|
|
|
|
It is important to note, however, that shadow registers aren't preserved.
|
|
Therefore, shadow registers should only be used in code that doesn't call
|
|
routines or that call a routine that explicitly states that it preserves
|
|
shadow registers.
|
|
|
|
Another important note is that routines returning success with Z generally don't
|
|
preserve AF: too complicated. But otherwise, AF is often preserved. For example,
|
|
register fiddling routines in core try to preserve AF.
|
|
|
|
## Z for success
|
|
|
|
The vast majority of routines use the Z flag to indicate success. When Z is set,
|
|
it indicates success. When Z is unset, it indicates error. This follows the
|
|
tradition of a zero indicating success and a nonzero indicating error.
|
|
|
|
Important note: only Z indicate success. Many routines return a meaningful
|
|
nonzero value in A and still set Z to indicate success.
|
|
|
|
In error conditions, however, most of the time A is set to an error code.
|
|
|
|
In many routines, this is specified verbosely, but it's repeated so often that
|
|
I started writing it in short form, "Z for success", which means what is
|
|
described here.
|
|
|
|
## Stack management
|
|
|
|
Keeping the stack "balanced" is a big challenge when writing assembler code.
|
|
Those push and pop need to correspond, otherwise we end up with completely
|
|
broken code.
|
|
|
|
The usual "push/pop" at the beginning and end of a routine is rather easy to
|
|
manage, nothing special about them.
|
|
|
|
The problem is for the "inner" push and pop, which are often necessary in
|
|
routines handling more data at once. In those cases, we walk on eggshells.
|
|
|
|
A naive approach could be to indent the code between those push/pop, but indent
|
|
level would quickly become too big to fit in 80 chars.
|
|
|
|
I've tried ASCII art in some places, where comments next to push/pop have "|"
|
|
indicating the scope of the push/pop. It's nice, but it makes code complicated
|
|
to edit, especially when dense comments are involved. The pipes have to go
|
|
through them.
|
|
|
|
Of course, one could add descriptions next to each push/pop describing what is
|
|
being pushed, and I do it in some places, but it doesn't help much in easily
|
|
tracking down stack levels.
|
|
|
|
So, what I've started doing is to accompany each "non-routine" (at the
|
|
beginning and end of a routine) push/pop with "--> lvl X" and "<-- lvl X"
|
|
comments. Example:
|
|
|
|
push af ; --> lvl 1
|
|
inc a
|
|
push af ; --> lvl 2
|
|
inc a
|
|
pop af ; <-- lvl 2
|
|
pop af ; <-- lvl 1
|
|
|
|
I think that this should do the trick, so I'll do this consistently from now on.
|
|
|
|
## String length
|
|
|
|
Pretty much every routine expecting a string have no provision for a string
|
|
that doesn't have null termination within 0xff bytes. Treat strings of such
|
|
lengths with extra precaution and distrust proper handling of existing routines
|
|
for those strings.
|
|
|
|
[zasm]: ../apps/zasm/README.md
|