collapseos/apps/zasm/README.md

z80 assembler

This is probably the most critical part of the Collapse OS project because it ensures its self-reproduction.

Running on a "modern" machine

To be able to develop zasm efficiently, libz80 is used to run zasm on a modern machine. The code lives in emul and ran be built with make, provided that you have a copy libz80 living in emul/libz80.

The resulting zasm binary takes asm code in stdin and spits binary in stdout.

Literals

There are decimal, hexadecimal and binary literals. A "straight" number is parsed as a decimal. Hexadecimal literals must be prefixed with 0x (0xf4). Binary must be prefixed with 0b (0b01100110).

Decimals and hexadecimal are "flexible". Whether they're written in a byte or a word, you don't need to prefix them with zeroes. Watch out for overflow, however.

Binary literals are also "flexible" (0b110 is fine), but can't go over a byte.

There is also the char literal ('X'), that is, two qutes with a character in the middle. The value of that character is interpreted as-is, without any encoding involved. That is, whatever binary code is written in between those two quotes, it's what is evaluated. Only a single byte at once can be evaluated thus. There is no escaping. ''' results in 0x27. You can't express a newline this way, it's going to mess with the parser.

Then comes our last literal, the string literal. It's a chain of characters surrounded by double quotes. Example: "foo". This literal can only be used in the .db directive and is equivalent to each character being single-quoted and separated by commas ('f', 'o', 'o'). No null char is inserted in the resulting value (unlike what C does).

Labels

Lines starting with a name followed : are labeled. When that happens, the name of that label is associated with the binary offset of the following instruction.

For example, a label placed at the beginning of the file is associated with offset 0. If placed right after a first instruction that is 2 bytes wide, then the label is going to be bound to 2.

Those labels can then be referenced wherever a constant is expected. They can also be referenced where a relative reference is expected (jr and djnz).

Labels can be forward-referenced, that is, you can reference a label that is defined later in the source file or in an included source file.

Labels starting with a dot (.) are local labels: they belong only to the namespace of the current "global label" (any label that isn't local). Local namespace is wiped whenever a global label is encountered.

Local labels allows reuse of common mnemonics and make the assembler use less memory.

Global labels are all evaluated during the first pass, which makes possible to forward-reference them. Local labels are evaluated during the second pass, but we can still forward-reference them through a "first-pass-redux" hack.

Labels can be alone on their line, but can also be "inlined", that is, directly followed by an instruction.

Constants

The .equ directive declares a constant. That constant's argument is an expression that is evaluated right at parse-time.

Constants are evaluated during the second pass, which means that they can forward-reference labels.

However, they cannot forward-reference other constants.

Expressions

Wherever a constant is expected, an expression can be written. An expression is a bunch of literals or symbols assembled by operators. For now, only +, - and * operators are supported. No parenthesis yet.

Expressions can't contain spaces.

The Program Counter

The $ is a special symbol that can be placed in any expression and evaluated as the current output offset. That is, it's the value that a label would have if it was placed there.

The Last Value

Whenever a .equ directive is evaluated, its resulting value is saved in a special "last value" register that can then be used in any expression. This is very useful for variable definitions and for jump tables.

Includes

The .inc directive is special. It takes a string literal as an argument and opens, in the currently active filesystem, the file with the specified name.

It then proceeds to parse that file as if its content had been copy/pasted in the includer file, that is: global labels are kept and can be referenced elsewhere. Constants too. An exception is local labels: a local namespace always ends at the end of an included file.

There an important limitation with includes: only one level of includes is allowed. An included file cannot have an .inc directive.

Directives

.db: Write bytes specified by the directive directly in the resulting binary. Each byte is separated by a comma. Example: .db 0x42, foo

.dw: Same as .db, but outputs words. Example: .dw label1, label2

.equ: Binds a symbol named after the first parameter to the value of the expression written as the second parameter. Example: .equ foo 0x42+'A'

If the symbol specified has already been defined, no error occur and the first value defined stays intact. This allows for "user override" of programs.

.fill: Outputs the number of null bytes specified by its argument, an expression. Often used with $ to fill our binary up to a certain offset. For example, if we want to place an instruction exactly at byte 0x38, we would precede it with .fill 0x38-$.

The maximum value possible for .fill is 0xd000. We do this to avoid "overshoot" errors, that is, error where $ is greater than the offset you're trying to reach in an expression like .fill X-$ (such an expression overflows to 0xffff).

.org: Sets the Program Counter to the value of the argument, an expression. For example, a label being defined right after a .org 0x400, would have a value of 0x400. Does not do any filling. You have to do that explicitly with .fill, if needed. Often used to assemble binaries designed to run at offsets other than zero (userland).

.out: Outputs the value of the expression supplied as an argument to ZASM_DEBUG_PORT. The value is always interpreted as a word, so there's always two out instruction executed per directive. High byte is sent before low byte. Useful or debugging, quickly figuring our RAM constants, etc. The value is only outputted during the second pass.

.inc: Takes a string literal as an argument. Open the file name specified in the argument in the currently active filesystem, parse that file and output its binary content as is the code has been in the includer file.

.bin: Takes a string literal as an argument. Open the file name specified in the argument in the currently active filesystem and outputs its contents directly.

Undocumented instructions

zasm doesn't support undocumented instructions such as the ones that involve using IX and IY as 8-bit registers. We used to support them, but because this makes our code incompatible with Z80-compatible CPUs such as the Z180, we prefer to avoid these in our code.