Nox ASM

Nox ASM is an assembler created for the fictional 8/16bit cpu, Nox CPU.

Syntax

Nox ASM syntax is loosely based on other 8-bit CPU assemblers, especially 6502's. It was designed to be as simple as possible while retaining readability and allowing the user to use all Nox CPU opcodes.

Nox ASM syntax is case-insensitive, meaning:

push a b == PUSH a b == push A B ect.

Special tokens

• $ - indicates a data stream. It can be a text, byte or multiple 8 or 16 bit values.

$ 0x1 0x23 0x02
or
$ "This is an ASCII text"
or
$ "This is an ASCII text that is followed by some values" 0x1234 0xde 0xad 0xbe 0xef
or 
$ 0x1 "Text" 0x2
etc.

• & - indicates an absolute addressing mode used in some instructions. By default, all calls involving labels are absolute. It is also used in indirect mode with the HLI register.

push 0x1234 AB  // This will push the value `0x1234` into `AB`
push &0x1234 AB // This will push the value read from address `0x1234` (and `0x1235`) into `AB`

jmp loop_end // This does not need the `&` symbol

PUSH 0xf000 HLI // `HLI` will now contain `0xf000`
INC HLI // `HLI` == `0xf001`
PUSH &HLI A // a value from `0xf001` will be pushed into `A`

• // - indicates the start of the comment. All following tokens are ignored

PUSH A B // This is a comment
// PUSH B A <= line is ignored completely

• > - indicates the absolute memory position of the following token. By default, the addressing starts at 0x0000

PUSH A B // this is on 0x0000

> 0x1234
PUSH B A // this is on 0x1234
ADD 0x12 A // this is on 0x1235

> 0xffff
HALT // this is the last byte

• * - indicates the memory location of the label

PUSH text AB // This pushes the value at `0x1234` to `AB`
PUSH *text HLI // This pushes the `0x1234` to `HLI`

> 0x1234
text:
$ "Some text"

Labels

Declaring a label is done by using any unique text (no whitespaces) followed by ":"

i_am_a_valid_label:
pop: // still valid
this:is:valid:too:
this is not a valid label:

To declare label at a specific address, use > token:

> 0x1234
i_am_at_0x1234:

JMP i_am_at_0x1234 // equal to JMP &0x1234

> 0xffff
i_am_the_last_byte:

Since the labels are assigned at second assembly pass, they do not need to be declared before the usage:

jmp this_is_not_yet_declared

this_is_not_yet_declared:
    POP A B

Instructions

For full list of all instructions along with the cycles count and flag changes, please see a full list of all Nox CPU opcodes