Although Apple's Super Serial Card can be used from Applesoft Basic, it is
often desirable to use machine language to increase the speed with which
characters are sent and received. The assembler program below illustrates a
method of communicating with another Apple computer through the Super Serial
Card. You may use this routine as a starting point for your own program.
On page 291 of the Apple IIe Reference manual and on pages 261 to 265 of the
Apple IIc Reference Manual there are listsof the registers and entry points
used by routines resident in the Super Serial Card. The equates in the program
below use these locations, as well as input/output hooks found in the Apple II
family of computers.
The initialization routine (INIT) stores the address of the Super Serial Card's
initialization routine in CSW (the Apple II monitor character output hook).
This activates the card for output by jumping to COUT. Following this, DOS or
Prodos hooks are reinstalled.
The OUTput routine checks the 6551 status port bit 4. If this is equal to zero,
the previous character has not yet been sent, so we must check the status byte
again until that register is clear. When the value in bit 4 becomes one, the
6551 is ready to send another character. To accomplish this, simply store the
data in the transmit register (TDREG) of the chip.
Bit 3 of the status port is checked by the INput routine. If this bit is zero,
the program either loops continuously or returns to the calling program,
depending on the state of the return flag found in location $FF. If bit 3 is
one, a character is waiting at the input port, and the character is then read
from the read register (RDREG) of the 6551.
The DEMO portion of this program calls the INIT routine, and sends each letter
of the alphabet to the connected device. After each character is sent, the
program waits to see if a response has been received from the external device.
If a character is waiting, the program ends.
1 * Super Serial Card - Demo of accessing it through machine language
10
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11 ORG $2000
12 COUT EQU $FDED ; CHARACTER OUT IN MONITOR
13 CSWL EQU $36 ; OUTPUT HOOK
14 CSWH EQU $37
15 WAIT EQU $FCA8 ; MONITOR ROUTINE TO WAIT
16 ;
17 ; SSC EQUATES
18 ;
19 DIPSW1 EQU $C081 ; +N0 DIPSWITCH BLOCK 1
20 DIPSW2 EQU $C082 ; +N0 DIPSWITCH BLOCK 2
21 TDREG EQU $C088 ; +N0 6551 DATA REGISTER
22 RDREG EQU $C088 ; +N0 6551 DATA REGISTER
23 STATUS EQU $C089 ; +N0 6551 STATUS REGISTER
24 RESET EQU $C089 ; +N0 6551 SOFTWARE RESET
25 COMMAND EQU $C08A ; +N0 6551 COMMAND REG
26 CONTROL EQU $C08B ; +N0 6551 CONTROL REG
27 ;
28 START JMP DEMO ; SKIP AROUND ALL THE SUBROUTINES
29 ;
30 ; USE THE SSC FIRMWARE TO INITIALIZE THE 6551.
31 ;
32 INIT LDA CSWL ; STORE THE CURRENT CSW
33 PHA ; SO THAT WE DO NOT DISCONNECT
34 LDA CSWH ; DOS OR PRODOS
35 PHA
36 LDA #$00 ; STORE $Cs00 IN CSW
37 STA CSWL
38 STX CSWH ; THIS ALREADY CONTAINS $Cs
39 LDA #$00
40 JSR COUT ; JUMP TO COUT TO INIT THE CARD
41 PLA
42 STA CSWH ; RESTORE THE DOS OR PRODOS
43 PLA ; HOOKS AND THEN RETURN
44 STA CSWL
45 RTS
46 ;
47 ; OUTPUT A CHARACTER TO 6551
48 ;
49 OUT PHA ; STORE DATA ON STACK
50 OLP LDA STATUS,Y ; CHECK BIT 4 OF STATUS BYTE
51 AND #$10 ; TO SEE IF IT'S OK TO SEND
52 BEQ OLP ; CHARACTER WAITING TO GO OUT
53 PLA ; GET DATA BACK FROM STACK
54 STA TDREG,Y ; AND OUTPUT THE CHARACTER
55 RTS
56 ;
57 ; INPUT A CHARACTER FROM 6551
58 ;
59 IN LDA STATUS,Y
60 AND #$08 ; BIT 3 OF STATUS
61 BEQ INTST ; NO CHAR WAITING TO BE RECEIVED
62 LDA RDREG,Y ; GET THE INPUT FROM 6551
63 RTS
64 INTST LDA $FF ; CHECK RETURN FLAG
65 BNE IN ; IF NOT 0 THEN WAIT FOR INPUT
66 RTS ; IF ZERO, DON'T WAIT
67 ;
68 ; BEGIN THE DEMO PROGRAM
69 ;
70 DEMO LDY #$10 ; Y CONTAINS $s0 - DEMO USES SLOT 1
71 LDX #$C1 ; LOAD X WITH $Cs
72 JSR INIT ; INIT THE CARD
73 LDA #$FF ; SET RETURN FLAG FOR INPUT
74 STA $FF ; FF MEANS WAIT FOR CHAR
75 JSR IN ; INPUT A CHARACTER - SEE ABOVE
76 OLOOP LDX #$41 ; OUTPUT THE ASCII CODES
77 OLP1 TXA ; FROM A-Z TO THE SSC. IT WILL STOP
78 JSR OUT ; WHEN THE SSC RECEIVES A CHAR.
79 LDA #$80 ; DELAY BETWEEN CHARACTERS
80 JSR WAIT ; TO ALLOW TIME FOR INPUT.
81 LDA #$00
82 STA $FF ; RETURN IF NO CHARS WAITING
83 JSR IN ; CHECK FOR A CHARACTER
84 BNE ALLDONE ; THEY SENT SOMETHING - WE END
85 INX
86 CPX #$5B ; THE LETTER 'Z'
87 BNE OLP1
88 LDA #$0D
89 JSR OUT ; SEND A CARRIAGE RETURN
90 JMP OLOOP ; BEGIN THE ALPHABET AGAIN
91 ALLDONE RTS ; END ROUTINE