AMOS BASIC tokenized file

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File Format
Name AMOS BASIC tokenized file
Extension(s) .amos
Released 1990

AMOS BASIC is a family of BASIC dialects for the Amiga computer. They were written by François Lionet, who also wrote AMOS's predecessor, STOS BASIC for the Atari ST (see STOS memory bank).

There are several versions of AMOS published: AMOS The Creator, Easy AMOS and AMOS Professional.

AMOS has its own integrated development environment, and it uses its own custom file formats for everything, from source code to graphics and sound.



AMOS is an interpreted BASIC dialect where code is edited and run in an integrated development environment. Every time the programmer finishes editing a line of code, it is immediately parsed into tokens. For example, typing procedure foobar and pressing the return key will change the line into Procedure FOOBAR.

Before a program can run, it will be tested to ensure it is free of syntax errors. Source code can be saved even if it is untested or fails testing, but AMOS includes a tested flag in the saved file. This is used by external software, for example the AMOS Compiler will refuse to compile an untested source code file.


AMOS tokens are split between instructions in the core language and instructions in extensions.

Extensions are external files, written in 68000 assembler, which begin with a token table listing all the instructions they add to the language.

Each extension is intended to be loaded into a specific slot. AMOS has 25 slots for extensions. The configuration of extensions and their slots are saved in AMOS's global config file.

To load other people's source code, AMOS needs to be configured with the same versions of the same extensions that they used, in the same slots.


In order to work with multimedia such as pictures and music, AMOS has the concept of a bank or memory bank (see AMOS Memory Bank#Disambiguation for a note about terminology). An AMOS program can have up to 15 banks. For example, you can load several pieces of music into different banks, and then identify which one you want to play by a number: Track Play 5 will play music in bank 5. Or you could load a packed picture into bank 4 and say Unpack 4 to 0, which will unpack the picture onto screen 0.

While you can load anything into any bank, some instructions can only take their data from specific bank numbers. Bank 1 is used for Sprites, which are controlled with instructions beginning Sprite or Bob. Bank 2 is for Icons, which are controlled with instructions beginning Icon. Bank 3 is used for music in AMOS's native music format.

If banks are in use while saving source code, the contents of the banks are included in the saved source code. This makes it easy to bundle code with the data it works on. The exception to this rule is banks created using the Reserve As Work instruction.

AMOS source code

All multi-byte integer values are in big-endian (Motorola) format.

AMOS source code is stored in a file with the extension .AMOS. It has the following structure:

Section Length
Header identifying which version of AMOS saved the file
  • "AMOS Pro111v\0\0\0\0"
  • "AMOS Pro101V\0\0\0\0" (AMOS Professional, source tested)
  • "AMOS Pro101v\0\0\0\0" (AMOS Professional, source not tested)
  • "AMOS Basic V134 " (AMOS Pro compatible with AMOS 1.3, source tested)
  • "AMOS Basic v134 " (AMOS Pro compatible with AMOS 1.3, source not tested)
  • "AMOS Basic V1.3 " (AMOS The Creator v1.3, source tested)
  • "AMOS Basic v1.3 " (AMOS The Creator v1.3, source not tested)
  • "AMOS Basic V1.23"
  • "AMOS Basic V1.00" (AMOS The Creator v1.0 – v1.2, source tested)
  • "AMOS Basic v1.00" (AMOS The Creator v1.0 – v1.2, source not tested)
16 bytes
Length in bytes of tokenized BASIC code to follow 4 bytes
Tokenized BASIC code varies
AMOS AmBs segment ASCII identifier "AmBs" 4 bytes
Count of AMOS banks to follow (0–16) 2 bytes
AMOS banks. Each bank's length must be individually determined. varies

Tokenised BASIC code

Tokenised BASIC code is a sequence of tokenised lines. Each tokenised line has the following format:

Field Length
Length of this line in words (2 bytes), including this byte. To get the length of the line in bytes, double this value 1 byte
Indent level of this line. Prefix indent level + 1 spaces at the beginning of the line, or no spaces if the value is less than 2 1 byte
Sequence of tokens. Each token is at least two bytes, and all tokens are rounded to to a multiple of two bytes. Each token is individually sized. The tokens always end with a compulsory null token varies

Some tokens have special size rules, but most are exactly 2 bytes in size.

Each token starts with a signed 16-bit number. Token values between 0x0000 and 0x004E have special printing and size rules, all other tokens are a signed offset into AMOS's internal token table. The instruction name in the internal token table is what should be printed.

Specially printed tokens

Token Type Interpretation
0x0000 null token Marks the end of line. Always 2 bytes long
0x0006 Variable reference, e.g. Print XYZ
  • 2 bytes: token
  • 2 bytes: unknown purpose
  • 1 byte: length of ISO-8859-1 string for the variable or label name
  • 1 byte: flags, for tokens 0x0006, 0x0012 and 0x0018:
    • bit 1 set: this is a floating point reference, e.g. XYZ#
    • bit 2 set: this is a string reference, e.g. XYZ$
  • variable length: ISO-8859-1 string, with the above given length. The string is null terminated and its length is rounded up to a multiple of two
0x000C Label, e.g. XYZ: or 190 at the start of a line
0x0012 Procedure call reference, e.g. XYZ["hello"]
0x0018 Label reference, e.g. Goto XYZ
0x0026 String with double quotes, e.g. "XYZ"
  • 2 bytes: token
  • 2 bytes: length of ISO-8859-1 string to follow
  • variable length: ISO-8859-1 string, with the above given length. The string is null terminated and its length is rounded up to a multiple of two
0x002E String with single quotes, e.g. 'XYZ'
0x001E Binary integer value, e.g. %100101
  • 2 bytes: token
  • 4 bytes: the integer value
0x0036 Hexidecimal integer value, e.g. $80FAA010
0x003E Decimal integer value, e.g. 1234567890
0x0046 Floating point value, e.g. 3.1452
  • 2 bytes: token
  • 4 bytes: the single-precision floating point value.
    • bits 31–8: mantissa (24 bits)
    • bit 7: sign bit. Positive if 0, negative if 1
    • bits 6–0: exponent

An exponent of 0 means 0.0, regardless of mantissa. Counting from MSB (23) to LSB (0), each bit set in the mantissa is 2^(mantissa bit + exponent − 88)

0x004E Extension instruction
  • 2 bytes: token
  • 1 byte: extension slot (1 to 26)
  • 1 byte: unused
  • 2 bytes: signed 16-bit offset into extension's token table

Specially sized tokens

Token Type Interpretation
0x064A Rem
  • 2 bytes: token (0x064A or 0x0652)
  • 1 byte: unused
  • 1 byte: length of ISO-8859-1 string to follow
  • variable length: ISO-8859-1 string, with the above-given length. The string is null terminated and its length is rounded up to a multiple of two. The string should be printed after the remark token.
0x0652 '
0x023C For
  • 2 bytes: token
  • 2 bytes: unknown purpose
0x0250 Repeat
0x0268 While
0x027E Do
0x02BE If
0x02D0 Else
0x0404 Data
0x0290 Exit If
  • 2 bytes: token
  • 4 bytes: unknown purpose
0x029E Exit
0x0316 On
0x0376 Procedure
  • 2 bytes: token
  • 4 bytes: number of bytes to corresponding End Proc line
(start of line + 8 + above = start of End Proc line)
(start of line + 8 + 6 + above = line after End Proc line)
  • 2 bytes: part of seed for encryption
  • 1 byte: flags
    • bit 7: if set, procedure is folded
    • bit 6: if set, procedure is locked and shouldn't be unfolded
    • bit 5: if set, procedure is currently encrypted
    • bit 4: if set, procedure contains compiled code and not tokens
  • 1 byte: part of seed for encryption

Encrypted procedures

If you should find a procedure (0x0376) token with the "is encrypted" bit set, run this C function on the code and it will decrypt the contents of the procedure.

/* fetches a 4-byte integer in big-endian format */
#define EndGetM32(a)  ((((a)[0])<<24)|(((a)[1])<<16)|(((a)[2])<<8)|((a)[3]))
/* fetches a 2-byte integer in big-endian format */
#define EndGetM16(a)  ((((a)[0])<<8)|((a)[1]))

void decrypt_procedure(unsigned char *src) {
    unsigned char *line, *next, *endline;
    unsigned int key, key2, key3, size;

    /* ensure src is a pointer to a line with the PROCEDURE token on it */
    if (EndGetM16(&src[2]) != 0x0376) return;

    /* do not operate on compiled procedures */
    if (src[10] & 0x10) return;

    /* size + 8 + 6 is the start of the line after END PROC */
    size = EndGetM32(&src[4]);
    endline = &src[size + 8 + 6];
    line = next = &src[src[0] * 2];

    /* initialise encryption keys */
    key = (size << 8) | src[11];
    key2 = 1;
    key3 = EndGetM16(&src[8]);

    while (line < endline) {
        line = next;
        next = &line[line[0] * 2];

        /* decrypt one line */
        for (line += 4; line < next;) {
            *line++ ^= (key >> 8) & 0xFF;
            *line++ ^=  key       & 0xFF;
            key = (key & 0xFFFF0000) | ((key+key2) & 0x0000FFFF);
            key2 += key3;
            key = (key >> 1) | (key << 31);
    src[10] ^= 0x20; /* toggle "is encrypted" bit */

AMOS Banks

AMOS banks can be found either included with AMOS source code, saved individually on disk, where they typically have the file extension .ABK.

AMOS allows for 15 banks in an program. Each bank can be located in "chip" memory, which is accessible to the Amiga's custom graphics and sound processors, or it can be located in "fast" memory, which is only accessible to the CPU.

Banks are identified by their first four bytes. They are either Sprite/Icon banks (using the ASCII identifier AmSp or AmIc), or they are "normal" memory banks, which covers all other possible bank formats (using the identifier AmBk).

AMOS Sprite/Icon Bank format

Refer to AMOS Sprite Bank. See also AMOS Icon Bank.

AMOS Memory Bank format

Refer to the main article: AMOS Memory Bank.

Sample files

Links and references

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