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188 lines
8.0 KiB
Markdown
188 lines
8.0 KiB
Markdown
# z80 assembler
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This is probably the most critical part of the Collapse OS project because it
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ensures its self-reproduction.
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## Invocation
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`zasm` is invoked with 2 mandatory arguments and an optional one. The mandatory
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arguments are input blockdev id and output blockdev id. For example, `zasm 0 1`
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reads source code from blockdev 0, assembles it and spit the result in blockdev
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1.
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Input blockdev needs to be seek-able, output blockdev doesn't need to (zasm
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writes in one pass, sequentially.
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The 3rd argument, optional, is the initial `.org` value. It's the high byte of
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the value. For example, `zasm 0 1 4f` assembles source in blockdev 0 as if it
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started with the line `.org 0x4f00`. This also means that the initial value of
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the `@` symbol is `0x4f00`.
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## Running on a "modern" machine
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To be able to develop zasm efficiently, [libz80][libz80] is used to run zasm
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on a modern machine. The code lives in `emul` and ran be built with `make`,
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provided that you have a copy libz80 living in `emul/libz80`.
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The resulting `zasm` binary takes asm code in stdin and spits binary in stdout.
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## 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 qutes 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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Then comes our last literal, the string literal. It's a chain of characters
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surrounded by double quotes. Example: `"foo"`. This literal can only be used
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in the `.db` directive and is equivalent to each character being single-quoted
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and separated by commas (`'f', 'o', 'o'`). No null char is inserted in the
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resulting value (unlike what C does).
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## Labels
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Lines starting with a name followed `:` are labeled. When that happens, the
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name of that label is associated with the binary offset of the following
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instruction.
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For example, a label placed at the beginning of the file is associated with
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offset 0. If placed right after a first instruction that is 2 bytes wide, then
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the label is going to be bound to 2.
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Those labels can then be referenced wherever a constant is expected. They can
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also be referenced where a relative reference is expected (`jr` and `djnz`).
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Labels can be forward-referenced, that is, you can reference a label that is
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defined later in the source file or in an included source file.
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Labels starting with a dot (`.`) are local labels: they belong only to the
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namespace of the current "global label" (any label that isn't local). Local
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namespace is wiped whenever a global label is encountered.
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Local labels allows reuse of common mnemonics and make the assembler use less
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memory.
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Global labels are all evaluated during the first pass, which makes possible to
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forward-reference them. Local labels are evaluated during the second pass, but
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we can still forward-reference them through a "first-pass-redux" hack.
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Labels can be alone on their line, but can also be "inlined", that is, directly
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followed by an instruction.
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## Constants
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The `.equ` directive declares a constant. That constant's argument is an
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expression that is evaluated right at parse-time.
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Constants are evaluated during the second pass, which means that they can
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forward-reference labels.
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However, they *cannot* forward-reference other constants.
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When defining a constant, if the symbol specified has already been defined, no
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error occur and the first value defined stays intact. This allows for "user
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override" of programs.
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It's also important to note that constants always override labels, regardless
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of declaration order.
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## Expressions
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Wherever a constant is expected, an expression can be written. An expression
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is a bunch of literals or symbols assembled by operators. For now, only `+`, `-`
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and `*` operators are supported. No parenthesis yet.
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Expressions can't contain spaces.
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## The Program Counter
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The `$` is a special symbol that can be placed in any expression and evaluated
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as the current output offset. That is, it's the value that a label would have if
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it was placed there.
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## The Last Value
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Whenever a `.equ` directive is evaluated, its resulting value is saved in a
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special "last value" register that can then be used in any expression. This
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last value is referenced with the `@` special symbol. This is very useful for
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variable definitions and for jump tables.
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Note that `.org` also affect the last value.
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## Includes
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The `.inc` directive is special. It takes a string literal as an argument and
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opens, in the currently active filesystem, the file with the specified name.
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It then proceeds to parse that file as if its content had been copy/pasted in
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the includer file, that is: global labels are kept and can be referenced
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elsewhere. Constants too. An exception is local labels: a local namespace always
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ends at the end of an included file.
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There an important limitation with includes: only one level of includes is
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allowed. An included file cannot have an `.inc` directive.
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## Directives
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**.db**: Write bytes specified by the directive directly in the resulting
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binary. Each byte is separated by a comma. Example: `.db 0x42, foo`
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**.dw**: Same as `.db`, but outputs words. Example: `.dw label1, label2`
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**.equ**: Binds a symbol named after the first parameter to the value of the
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expression written as the second parameter. Example:
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`.equ foo 0x42+'A'`. See "Constants" above.
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**.fill**: Outputs the number of null bytes specified by its argument, an
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expression. Often used with `$` to fill our binary up to a certain
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offset. For example, if we want to place an instruction exactly at
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byte 0x38, we would precede it with `.fill 0x38-$`.
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The maximum value possible for `.fill` is `0xd000`. We do this to
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avoid "overshoot" errors, that is, error where `$` is greater than
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the offset you're trying to reach in an expression like `.fill X-$`
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(such an expression overflows to `0xffff`).
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**.org**: Sets the Program Counter to the value of the argument, an expression.
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For example, a label being defined right after a `.org 0x400`, would
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have a value of `0x400`. Does not do any filling. You have to do that
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explicitly with `.fill`, if needed. Often used to assemble binaries
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designed to run at offsets other than zero (userland).
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**.out**: Outputs the value of the expression supplied as an argument to
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`ZASM_DEBUG_PORT`. The value is always interpreted as a word, so
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there's always two `out` instruction executed per directive. High byte
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is sent before low byte. Useful or debugging, quickly figuring our
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RAM constants, etc. The value is only outputted during the second
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pass.
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**.inc**: Takes a string literal as an argument. Open the file name specified
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in the argument in the currently active filesystem, parse that file
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and output its binary content as is the code has been in the includer
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file.
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**.bin**: Takes a string literal as an argument. Open the file name specified
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in the argument in the currently active filesystem and outputs its
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contents directly.
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## Undocumented instructions
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`zasm` doesn't support undocumented instructions such as the ones that involve
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using `IX` and `IY` as 8-bit registers. We used to support them, but because
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this makes our code incompatible with Z80-compatible CPUs such as the Z180, we
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prefer to avoid these in our code.
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[libz80]: https://github.com/ggambetta/libz80
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