Apple II manual Address Content, $Ae, Inputnumbern, Printx

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

20INPUT"NUMBER";N

30X=USR(N)

40PRINTX

50GOTO 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,