Home > CBM > BASIC > Keywords > Usr
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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 | 1 | 2 |
| 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.
FN, FETCH, PEEK,
RREG, SWAP
DEF, DIM, LET
POINTER, POKE, SYS,
Variable Format
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