Commodore 128D - Page 339

USR
VAL
—Call user-defined subprogram
USR(X)
When this function is used, the program jumps to a machine lan
guage program whose starting point is contained in memory loca
tions 4633($1219) and 4634($121 A), (and 785($0311) and 786($0312)
for C64 mode). The parameter X is passed to the machine-language
program in the floating point accumulator. A value is returned to the
BASIC program through the calling variable. You must redirect the
value into a variable in your program in order to receive the value
back from the floating point accumulator. An ILLEGAL QUANTITY
ERROR results if you don't specify this variable. This allows the user
to exchange a variable between machine code and BASIC.
EXAMPLE:
10 POKE 4633,0
20 POKE 4634,19
30 A = USR(X)
40 PRINT A
Place starting location ($1300 = 4864:$00 = 0:$13 = 19) of machine
language routine in location 4633 and 4634. Line 30 stores the
returning value from the floating point accumulator. This example
assumes you have a machine language routine at $1300.
—Return the numeric value of a number string
VAL(X$)
This function converts the string X$ into a number which is the
inverse operation of STR$. The string is examined from the left-most
character to the right, for as many characters as are in recognizable
number format. If the Commodore 128 finds illegal characters, only
the portion of the string up to that point is converted. If no numeric
characters are present, VAL returns a 0.
EXAMPLE:
10 A$ = "120"
20 B$ = "365"
30 PRINT VAL (A$ + VAL B$)
RUN
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BASIC 7.0 ENCYCLOPEDIA—Basic Functions