IBM 5150 - Page 267

LOC OBJ
LINE SOURCE
H20 C3
5415 RET
;WE ARE FINISHED
5416 WRITE_BYTE ENDP
5417
1----- --- - -- --- -- -------------------- ----
FAZE
5418 WRITE_BIT
PROC
NEAR
5419
; PURPOSE:
5420
5421
TO WRITE A oAT A BI T TO CASSETTE
5422 CARRY FLAG CONTAINS DATA BIT
5423
I. E. IF SET DATA BIT IS A ONE
5424
IF ClEAR DATA BIT IS A ZERO
5425
5426
NOTE: TWO EDGES ARE WRITTEN PER BIT
5427
ONE BIT HAS 500 USEe BETWEEN EDGES
5428
FOR A 1000 USEC PERIOD (1 MllLISEC)
5429
5430
ZERO BIT HAS 250 USEC BETWEEN EDGES
5431
fOR A. SOD USEC PERIOD (.5 MIlLISEC)
5432 ; CARRY FLAG IS DATA BIT
5433
1---- ---------------------------------------
5434 ;ASSUME IT'S A
','
FAZE 88,6.004 5435
MOY AX.1l84
; SET AX TO NOMINAL ONE SIZE
FA31 7203 5436
JC
; JUMP IF ONE BIT
w,.
FA33 885002 5437 MOY
AX,59Z
; NO, SET TO NOMINAL ZERO SIZE
FA36 5438 10126:
; WRITE-BIT-AX
FA36 50 5439
PUSH AX
iWRITE BIT WITH PERIOD EQ TO VALUE AX
FA37 5440
10129:
FA37 E462 5441
IN AL,PORT_C
; INPUT TIMER_O
OUTPUT
FH9 2420 5442 AND AL,020H
FA38 74F,6.
5443
Jl WZ9
,lOOP TIll HIGH
FA30 5444
10130:
FA30
E462 5445
IN Al.PORT_C
;NOW WAIT TILL TIMER'S OUTPUT IS LOW
F,6.3F 2420
5446
AND
AL,020H
FA41 75FA
5447
JHZ W3.
5448
jRElOAD TIMER WITH PERIOD
5449 iFOR NEXT DATA BIT
..
FA43 58 5450 POP
;RESTQRE PERIOD COUNT
FA44 5451
10131: SET TIMER
FA44 E642 5452
,
OUT
"
042H,
AL
; SET LOW BYTE OF TIMER 2:
FA46 BAC4 5453 MOV
AL, AH
FA48 E642 5454 OUT 042H.
AL
; SET HIGH BHE OF TIME~ 2
FA4A C3
5455 RET
545~
WRITE_BIT
ENDP
""Ii'
" ~;~=~;~---------;;~~----~;~;------- -- -----
FA48
, 5458
.. 5459
UPDATE CRC REGISTER WITH NEXT DATA BIT
,
,
5460
.. 5461 CRe IS USED TO DETECT READ ERRORS
:\
5462
•
." ,
5463
ASSUMES DATA BIT IS IN CARRY
REG AX IS MODIFIED
FLAGS ARE MODIFIED
5467
F,6.48 ,6.16900 5468
MOY
5469
j THE FOLLOWING INSTUCTIONS
5470
JWILL SET
THE OVERFLOW FLAG
5471
j IF ~ARRY AND MS BIT OF CRe
5472
JARE UNEQUAL
FA4E 0108
5473
RCR
AX.1
FA50
0100
5474
RCL
AX,I
1A52 F8 5475 eLe
;CLEAR CARRY
FA53 7104 5476 JNO
,SKIP IF NO OVERflOW
5477
;IF DATA BIT XORED WITH
5478
; CRe REG BIT 15 IS ONE
FA55 351008 5479 xtJR AX.D810H
;
THEN XOR CRC REG WITH
5480
; oa10H
FA58 F9
5461
STC
;SET CARRY
F,6.59 5482 10132:
W"
AX.,
FA59 0100 5483 RCL
iROTATE CARRY (DATA BIT)
5484
jlNTO CRe REG
FA5B ,6.36900
5485
Mev
CRC_REG.AX
;UPDATE CRC_REG
FASE C3 5486 RET
; FINISHED
5487
CRe_GEN
ENDP
5488
;--------------------------------------------
F,6.5F 5469 PROC NEAR
; START TAPE AtJO DELAY
5490
5491
; ------------------------ ------------ --------
A-74