collapseos/doc/glue-code.md
Virgil Dupras 7274dccbe7 Move ASCII consts to ascii.h
And made them shorter in name. The new ascii.h file allow reuse in userspace
code.
2019-11-13 20:38:06 -05:00

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# Writing the glue code
Collapse OS's kernel code is loosely knit. It supplies parts that you're
expected to glue together in a "glue code" asm file. Here is what a minimal
glue code for a shell on a Classic [RC2014][rc2014] with an ACIA link would
look like:
; The RAM module is selected on A15, so it has the range 0x8000-0xffff
.equ RAMSTART 0x8000
.equ RAMEND 0xffff
.equ ACIA_CTL 0x80 ; Control and status. RS off.
.equ ACIA_IO 0x81 ; Transmit. RS on.
jp init
; interrupt hook
.fill 0x38-$
jp aciaInt
.inc "err.h"
.inc "ascii.h"
.inc "core.asm"
.inc "parse.asm"
.equ ACIA_RAMSTART RAMSTART
.inc "acia.asm"
.equ STDIO_RAMSTART ACIA_RAMEND
.equ STDIO_GETC aciaGetC
.equ STDIO_PUTC aciaPutC
.inc "stdio.asm"
.equ SHELL_RAMSTART STDIO_RAMEND
.equ SHELL_EXTRA_CMD_COUNT 0
.inc "shell.asm"
init:
di
; setup stack
ld hl, RAMEND
ld sp, hl
im 1
call aciaInit
call shellInit
ei
jp shellLoop
Once this is written, building it is easy:
zasm < glue.asm > collapseos.bin
## Building zasm
Collapse OS has its own assembler written in z80 assembly. We call it
[zasm][zasm]. Even on a "modern" machine, it is that assembler that is used,
but because it is written in z80 assembler, it needs to be emulated (with
[libz80][libz80]).
So, the first step is to build zasm. Open `tools/emul/README.md` and follow
instructions there.
## Platform constants
The upper part of the code contains platform-related constants, information
related to the platform you're targeting. You might want to put it in an
include file if you're writing multiple glue code that targets the same machine.
In all cases, `RAMSTART` are necessary. `RAMSTART` is the offset at which
writable memory begins. This is where the different parts store their
variables.
`RAMEND` is the offset where writable memory stop. This is generally
where we put the stack, but as you can see, setting up the stack is the
responsibility of the glue code, so you can set it up however you wish.
`ACIA_*` are specific to the `acia` part. Details about them are in `acia.asm`.
If you want to manage ACIA, you need your platform to define these ports.
## Header code
Then comes the header code (code at `0x0000`), a task that also is in the glue
code's turf. `jr init` means that we run our `init` routine on boot.
`jp aciaInt` at `0x38` is needed by the `acia` part. Collapse OS doesn't dictate
a particular interrupt scheme, but some parts might. In the case of `acia`, we
require to be set in interrupt mode 1.
## Includes
This is the most important part of the glue code and it dictates what will be
included in your OS. Each part is different and has a comment header explaining
how it works, but there are a couple of mechanisms that are common to all.
### Defines
Parts can define internal constants, but also often document a "Defines" part.
These are constant that are expected to be set before you include the file.
See comment in each part for details.
### RAM management
Many parts require variables. They need to know where in RAM to store these
variables. Because parts can be mixed and matched arbitrarily, we can't use
fixed memory addresses.
This is why each part that needs variable define a `<PARTNAME>_RAMSTART`
constant that must be defined before we include the part.
Symmetrically, each part define a `<PARTNAME>_RAMEND` to indicate where its
last variable ends.
This way, we can easily and efficiently chain up the RAM of every included part.
### Tables grafting
A mechanism that is common to some parts is "table grafting". If a part works
on a list of things that need to be defined by the glue code, it will place a
label at the very end of its source file. This way, it becomes easy for the
glue code to "graft" entries to the table. This approach, although simple and
effective, only works for one table per part. But it's often enough.
For example, to define extra commands in the shell:
[...]
.equ SHELL_EXTRA_CMD_COUNT 2
#include "shell.asm"
.dw myCmd1, myCmd2
[...]
### Initialization
Then, finally, comes the `init` code. This can be pretty much anything really
and this much depends on the part you select. But if you want a shell, you will
usually end it with `shellLoop`, which never returns.
[rc2014]: https://rc2014.co.uk/
[zasm]: ../tools/emul/README.md
[libz80]: https://github.com/ggambetta/libz80