mirror of
https://github.com/hsoft/collapseos.git
synced 2024-11-17 09:28:06 +11:00
63473cc2e3
I'm about to change that structure and I don't want fs to be messed up by this. I'm proceeding step by step...
144 lines
4.6 KiB
Markdown
144 lines
4.6 KiB
Markdown
# 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.
|
|
|
|
jr init
|
|
|
|
; interrupt hook
|
|
.fill 0x38-$
|
|
jp aciaInt
|
|
|
|
init:
|
|
di
|
|
; setup stack
|
|
ld hl, RAMEND
|
|
ld sp, hl
|
|
im 1
|
|
call aciaInit
|
|
xor a
|
|
ld de, BLOCKDEV_SEL
|
|
call blkSel
|
|
call stdioInit
|
|
call shellInit
|
|
ei
|
|
jp shellLoop
|
|
|
|
#include "core.asm"
|
|
.equ ACIA_RAMSTART RAMSTART
|
|
#include "acia.asm"
|
|
.equ BLOCKDEV_RAMSTART ACIA_RAMEND
|
|
.equ BLOCKDEV_COUNT 1
|
|
#include "blockdev.asm"
|
|
; List of devices
|
|
.dw aciaGetC, aciaPutC, 0, 0
|
|
|
|
.equ STDIO_RAMSTART BLOCKDEV_RAMEND
|
|
#include "stdio.asm"
|
|
|
|
.equ SHELL_RAMSTART STDIO_RAMEND
|
|
.equ SHELL_EXTRA_CMD_COUNT 0
|
|
#include "shell.asm"
|
|
|
|
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
|