Apple II 사용자 설명서

Parameter (W), passed to a subroutine by USR(W), will be converted to floating-point accumulator

located at $A9.  The floating-point accumulator has the following format:

        ADDRESS         CONTENT

        $A9             Exponent + $81 ($80 if mantissa = 00)

        $AA-$AD         Mantissa, normalized so that Bit 7 of MSB is set.

                        $AA is MSB, $AD is LSB.

        $AE             Sign of mantissa

A parameter passed to an assembly language subroutine from BASIC can be truncated by the sub-

routine to a 2-byte integer and deposited in $AC (MSB) and $AD (LSB).  If the parameter is

greater than 32767 or less than -32768, an FC error will result.  The address of the subroutine

that converts a floating-point number to an integer is located in $B006, $B007.

A parameter passed to BASIC from an assembly language subroutine will be converted to floating-

point.  The address of the subroutine that performs this conversion is in $B008, $B009.  The

integer MSB ($AC) must be in the accumulator; the integer LSB ($AD) must be in the Y register.

Prior to executing USR, the starting address of the assembly language subroutine must be stored

in locations $04 (LSB) and $05 (MSB).  This is generally performed using the POKE command.

Note that more than one assembly language subroutine may be called from a BASIC program,

by changing the starting address in $04 and $05.

Figure F-1 is the listing for a BASIC program that calls an assembly language subroutine located at

$A00.  Here's what the BASIC program does:

    *   Line 10 - Stores the starting address of the assembly language subroutine ($A00) into

        locations $04 and $05, using POKE.

    *   Line 20 - Asks for a number "N".

    *   Line 30 - Calls the subroutine, with N as the parameter.

    *   Line 40 - Upon return from the subroutine, the BASIC program prints X, the parameter

        passed from the subroutine to the BASIC program.

    *   Line 50 - Loops back to get a new N

                ROCKWELL AIM 65

                <5>

                MEMORY SIZE? 2048

                WIDTH?

                 1518 BYTES FREE

                AIM 65 BASIC V1.1

                OK

                10 POKE 04,0: POKE 05

                ,10

                20 INPUT"NUMBER";N

                30 X=USR(N)

                40 PRINTX

                50 GOTO 20

Figure F-1.  BASIC Program That Calls Assembly Language Subroutine

The assembly language subroutine (Figure F-2) performs these operations:

    *   Prints the floating-point accumulator ($A9-$AE), using Monitor subroutines NUMA

        ($EA46), BLANK ($E83E) and CRLF ($E9F0),

    *   Converts the floating-point accumulator to an integer, using the subroutine at $BF00.

        The address $BF00 was found in locations $B006, $B007.  (Address $BF00 may vary

        with different versions of BASIC.  Be sure to check locations $B006 and $B007 for the

        correct address.)

    *   After conversion, the program again prints the floating point accumulator,