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112 lines
4.8 KiB
Markdown
112 lines
4.8 KiB
Markdown
# User applications
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This folder contains code designed to be "userspace" application. Unlike the
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kernel, which always stay in memory. Those apps here will more likely be loaded
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in RAM from storage, ran, then discarded so that another userspace program can
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be run.
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That doesn't mean that you can't include that code in your kernel though, but
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you will typically not want to do that.
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## Userspace convention
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We execute a userspace application by calling the address it's loaded into.
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This means that userspace applications must be assembled with a proper `.org`,
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otherwise labels in its code will be wrong.
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The `.org`, it is not specified by glue code of the apps themselves. It is
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expected to be set either in the `user.h` file to through `zasm` 3rd argument.
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That a userspace is called also means that an application, when finished
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running, is expected to return with a regular `ret` and a clean stack.
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Whatever calls the userspace app (usually, it will be the shell), should set
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HL to a pointer to unparsed arguments in string form, null terminated.
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The userspace application is expected to set A on return. 0 means success,
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non-zero means error.
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A userspace application can expect the `SP` pointer to be properly set. If it
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moves it, it should take care of returning it where it was before returning
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because otherwise, it will break the kernel.
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## Memory management
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Apps in Collapse OS are design to be ROM-compatible, that is, they don't write
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to addresses that are part of the code's address space.
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By default, apps set their RAM to begin at the end of the binary because in
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most cases, these apps will be ran from RAM. If they're ran from ROM, make sure
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to set `USER_RAMSTART` properly in your `user.h` to ensure that the RAM is
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placed properly.
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Applications that are ran as a shell (the "shell" app, of course, but also,
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possibly, "basic" and others to come) need a manual override to their main
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`RAMSTART` constant: You don't want them to run in the same RAM region as your
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other userspace apps because if you do, as soon as you launch an app with your
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shell, its memory is going to be overwritten!
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What you'll do then is that you'll reserve some space in your memory layout for
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the shell and add a special constant in your `user.h`, which will override the
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basic one (remember, in zasm, the first `.equ` for a given constant takes
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precedence).
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For example, if you want a "basic" shell and that you reserve space right
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after your kernel RAM for it, then your `user.h` would contain
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`.equ BAS_RAMSTART KERNEL_RAMEND`.
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You can also include your shell's code directly in the kernel by copying
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relevant parts of the app's glue unit in your kernel's glue unit. This is often
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simpler and more efficient. However, if your shell is a big program, it might
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run into zasm's limits. In that case, you'd have to assemble your shell
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separately.
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## Common features
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The folder `lib/` contains code shared in more than one apps and this has the
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effect that some concepts are exactly the same in many application. They are
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therefore sharing documentation, here.
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### Number literals
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There are decimal, hexadecimal and binary literals. A "straight" number is
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parsed as a decimal. Hexadecimal literals must be prefixed with `0x` (`0xf4`).
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Binary must be prefixed with `0b` (`0b01100110`).
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Decimals and hexadecimal are "flexible". Whether they're written in a byte or
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a word, you don't need to prefix them with zeroes. Watch out for overflow,
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however.
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Binary literals are also "flexible" (`0b110` is fine), but can't go over a byte.
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There is also the char literal (`'X'`), that is, two quotes with a character in
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the middle. The value of that character is interpreted as-is, without any
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encoding involved. That is, whatever binary code is written in between those
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two quotes, it's what is evaluated. Only a single byte at once can be evaluated
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thus. There is no escaping. `'''` results in `0x27`. You can't express a newline
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this way, it's going to mess with the parser.
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### Expressions
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An expression is a bunch of literals or symbols assembled by operators.
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Supported operators are `+`, `-`, `*`, `/`, `%` (modulo), `&` (bitwise and),
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`|` (bitwise or), `^` (bitwise xor), `{` (shift left), `}` (shift right).
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Bitwise operator always operate on the whole 16-bits.
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Shift operators break from the `<<` and `>>` tradition because the complexity
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if two-sized operator is significant and deemed not worth it. The shift
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operator shift the left operand X times, X being the right operand.
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There is no parenthesis support yet.
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Symbols have a different meaning depending on the application. In zasm, it's
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labels and constants. In basic, it's variables.
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Expressions can't contain spaces.
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Expressions can have an empty left operand. It will then be considered as 0.
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This allows signed integers, for example, `-42` to be expressed as expected.
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That form doesn't work well everywhere and is mostly supported for BASIC. In
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zasm, you're safer with `0-42`.
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