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| Keyword | Abbreviation | Token (hex) | Version(s) | Classification | | USR | U{Shift+S} | B7 | 1.0+ | Function |
USR ( value ) | Parameters | Type | Legal Value(s) | Default Value | Note(s) | | value | Numeric | depends of function definition | | |
| Returns | Type | Value(s) | Note(s) | | result | Floating-point | depends on function definition | |
Call a machine-language program to evaluate a custom user-function.
The USR function is the functional equivalant to the SYS command/statement. Another way to think of it, is as the machine-language equivalant of the BASIC (DEF)FN. In contrast to SYS, you pass a numeric value to be processed, not a memory address (the memory address of the machine-language program is a secret variable). In contrast to (DEF)FN, the definition is written is assembly/machine language, instead of BASIC. Also different from (DEF)FN is that you can have only one custom USR function (unless you keep POKE'ing different values, see below). Because it calls a machine-language program, this function is very non-portable. If no machine-langauge program has been "set up", then calling USR will generate ILLEGAL QUANTITY ERROR. To "set up" USR, poke the start address of a machine-language program (which should store its result in BASIC's floating-point accumulator) into the magic addresses shown below: | Machine | Address | | Low Byte | High Byte | | PET | ??? | ??? | | CBM-II | 3 | 4 | | VIC-20 | 1 | 2 | | Commodore 64 | 785 | 786 | | Commodore 16 | 1281 | 1282 | | Plus/4 | 1281 | 1282 | | Commodore 128 | 4633 | 4644 |
You will get TYPE MISMATCH ERROR with a string value. Because this is extemely machine-dependent, there are no examples.
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