titus-larsen-titus-1981-apple-interfacing

••
Addrss Byte
Da;a Syie
Mexacledmi !.ledm;;I rlexadedmal !lledmal
0300 768 - - MASK Byte Goes Here
0301
- - DATA Byte Goes Here
0302 770 - - ANSWER Found Here
0303 771 48 72 PHA Push Reg A
.. . 0304
0305
772
773
AD
00
173
0
LDA Load Reg A from
MASK location
I
0306
03 3
0307 775 2D
AND Reg A with DATA *
0308 776 01 1
0309
03 3
.. 030A 778 SD 141 STA Store result in
I 0308
02 2 ANSWER location
030C 780 03 3
030D 781 68 104 PLA Pul I Reg A back
. 030E 782 60 96 RTS Return to BASIC
769
774
777
779
*Substitute OOH, or 13 decimal, for an OR operation.
45
Three read/ write memory locations are used for the temporary
of the various data bytes, called MASK, BYTE, and AN­
The MASK location is loaded with the mask byte, and the
location is loaded with the byte that is to be operated on. After
the logical operation has taken place, the ANSWER location contains
result.
To use this routine, you need to load the MASK information into
768, and the DATA byte into address 769. You can use
operations to do this. Once this is done, you simply need to
the assembly-language subroutine, so that the operation is per­
·,,.,.. v. How do you do this
Calling an assembly-language subroutine from BASIC is not very
difficult. In the Apple computer, you simply need to put a three-byte
instruction in three successive locations, addresses 10, 11, and
or OB, and OC, in hexadecimal notation. Since our routine
at address 771, or 0303H, you need to put the following infor­
,,,ucivu in these three locations: a 76 in address 10, a 3 in address 11,
a 3 in address 12. Once you have loaded this ad.dress information
these three locations, you can access the assembly-language subwith
a USR function. In this case, you need to first load the
and BYTE information, and then use the USR function. This
shown in Example 4-3.
In this case, the value 32 is the mask byte, and 129 is the value that
to be ANDed with it. The Q fa a "dummy" variable that is required
the use of the USR function, and the value 5 is a "dummy" value
has no effect on the subroutine. You can use any variable for Q,