mk90.basic

MBX1 executable format

The file header and opcode encoding for bytecode ABI 1.

Byte offset	Length	Value
0	4	ASCII MBX1.
4	2	Bytecode ABI, 1.
6	2	Flags, 0.
8	2	Total file size, including header.
10	2	Variable count, 26.
12	2	Operand-stack limit, 16.
14	2	Reserved, 0.

Words are little-endian. The minimum accepted executable length is 17 bytes: the header and at least an instruction byte. The file-data checksum is the MKS2 inode checksum, not a field in this header. The entire file is checked before execution.

Encoding

Each opcode is one byte. Variable operands are zero-based, A=0 through Z=25. Immediate word operands and branch offsets are little-endian. Branch offsets count from the start of the file, so executable targets must be at least 16 and less than the total size.

LINE is an internal source-line marker used for error reporting; the source-language drawing LINE compiles to DRAW. KEY implements INKEY. STRING and PATH carry a length byte followed by data. All other opcodes have no inline operand unless listed below; their arguments come from the operand stack.

Opcode index

Decimal	Hex	Mnemonic	Inline operand
0	`00`	`END`	None
1	`01`	`LINE`	u16 source-line marker
2	`02`	`CONST`	u16 bit pattern
3	`03`	`LOAD`	u8 variable index
4	`04`	`STORE`	u8 variable index
5	`05`	`ADD`	None
6	`06`	`SUB`	None
7	`07`	`MUL`	None
8	`08`	`DIV`	None
9	`09`	`MOD`	None
10	`0A`	`NEG`	None
11	`0B`	`EQ`	None
12	`0C`	`NE`	None
13	`0D`	`LT`	None
14	`0E`	`LE`	None
15	`0F`	`GT`	None
16	`10`	`GE`	None
17	`11`	`AND`	None
18	`12`	`OR`	None
19	`13`	`NOT`	None
20	`14`	`PRINT`	None
21	`15`	`STRING`	u8 length, then literal bytes
22	`16`	`NL`	None
23	`17`	`JMP`	u16 byte offset
24	`18`	`JNZ`	u16 byte offset
25	`19`	`CALL`	u16 byte offset
26	`1A`	`RET`	None
27	`1B`	`FOR`	u8 variable index
28	`1C`	`NEXT`	u8 variable index
29	`1D`	`INPUT`	u8 variable index
30	`1E`	`CLS`	None
31	`1F`	`BEEP`	None
32	`20`	`WAIT`	None
33	`21`	`LOCATE`	None
34	`22`	`PSET`	None
35	`23`	`PRESET`	None
36	`24`	`DRAW`	None
37	`25`	`SOUND`	None
38	`26`	`TICKS`	None
39	`27`	`KEY`	None
40	`28`	`ABS`	None
41	`29`	`RND`	None
42	`2A`	`OPENR`	None
43	`2B`	`OPENW`	None
44	`2C`	`GET`	u8 variable index
45	`2D`	`PUT`	None
46	`2E`	`CLOSE`	None
47	`2F`	`EOF`	None
48	`30`	`HOUR`	None
49	`31`	`MINUTE`	None
50	`32`	`SECOND`	None
51	`33`	`PATH`	u8 length, then pathname bytes

Compatibility

MBX1 is a format for this VM. It is not ELF, PDP-11 a.out, a native instruction stream or a tokenized factory-BASIC cartridge. Header and bounds validation do not establish full semantic validity for manually edited bytecode. Use MBC from the matching release.