Section 2 - Commodore Computers

May, 1982, Issue 24
116 . COMPUTEI
COMPUTE! May. 1982. Issue 24
better way of detecting g errors. The most common
CCRC that is used is a 16 bit polynomial, defined as
(X + I 1 )*(X' )*(X* 15 + X + I). 1). By starting with a value e of
zero in the CRC and passing g all the (lie received data
through the CCRC routine. routine, when the final CRC
received passes through the routine, the final
result will be 0 (if {ifno errors were eencountered).
There is no single besl best way to transfer programs
via a modem, but bill there are some melhods
methods
which arc are better than olhers.
others.
*CPItH *C/V/W ;S is 1I a Iraril'Jllarlt trademark oj ofDigital Rt'search
Research
Computing A Checksum
Checksum generates a sum of the data that
is passed through it by adding (without any
carry) each byte with the sum of the previous
bytes. The checksum is initiallyy set to zero
aand the final result is sent to be compared
against the independently computed check­
sum atlhe the receiver computer. Since thee
checksum is ann eighl eight bit value. value, only a single
byte of data needs to be sent. It is quick and
easy to perform this with a computer. The
checksum method can onlyy reliably catch
single bit errors. Although it does do reasonably
well with wilh multibil Illultibit errors, the e percentages
can rapidly drop lo to the realm of or
coin toss odds.
To use the checksum program enter it
To use the checksum program enter it
with the data in the accumulator. The result
is saved in location CHKSUMC H for [i) r later use.
The location CHKSUMC I-I M should be cleared
to zero at the start of sending the data
to zero at the start o f sending the data
stream. stream,
8080 VERSION 6502 VERSION
CHKSUM PUSH PSW CHKSUM PHA
PUSH B
PHP
MOV
LDA LOA
ADD STA POP POP RET
C,A CHKSUM
C CHKSUM B PSW CLC
ADC STA PLP
PLA
RTS
CHKSUM
CHKSUM
This shortha nd version for th e 8080
This shorthand version for the 8080
assumes that at a register in the I CPU C does
not get destroyed by the calling program,
and that it is acceptable for the flags to be
destroyed.
not get destroyed by the calling program,
and that it is acceptable for the flags to be
d estroyed .
8080 VERSION
8080 VERSION
CHKSUM ADDC C
MOV C,A
MOVC,A
RET
RET
The CRC Method
eRC computes CRC-1 6 crcsum from the poly­
CRC computes CRC-16 crcsum from the poly
nomial nomiaI(X+l)*X"15 1)* X' + X X+l) + I)
A CRC method of error detection is far
supe superior to the checksum. By using a 16 Hi bit
value e for the sum instead or ofan eight bit
va value, e. a much improved detection capability
is achieved. By using a polynomial of th the e
proper type, the 16 bit va value can be used
ra far r more effectively as well. The provided
polynomial can detect errors or of up to 171 7
bits.
Since the CRCSUM generated is a
division remainder, r, a CRCSUMed CRCSUMcd data
sequence can be verified ed by running the
data through the CRC, and thenn running
the e previously obtained CCRCSUM M throughh
the CRC. Re. The resultant CRCSUMM should
be zero. When the CRCSUM itself is transmittemined,
, it should not be run through the e
trans-
CRC as this would disrupt the result. Also. Also,
the MSB (1-1) {H } must be run through h the
CRC first then followed by the LSB (L)
{ L }
whenn checking the CRCSUM.
To use li this routine, rOlltine, enter with the byte
to be CRCSUMed in A (accumulator). TThe
CRCSUMC is automatically tica y updated upon UPO Il
passing the data through this is routine.
8080 VERSION
6502 VERSION
CRC
CRC2 2
A CRC method of error detection is rar
bits.
Si nce the CRCSUM generated is a
C RC
CRCI
PUSH P PSW
PUSH PUS H B8
PUSH
H
MVI B,8 8,8
LHLD LHLO
CRCSUM
RCL
MOV
C,A
MOV
A,L
ADD AOO
A
MOV
L.A
MOV 1\-fOV
A,H
RAL
MOV
H.A
RAL
XRA
C
RRC
JNC
CRC2
MOV
A,H
XRI
80H
MOV H,A H.A
MOV A.L
MOV
XRI X IU
MOV
MOV
DCR OCR
JNZ
SHLD
POP
POP
POP
RET
ENO
END
A.L
051-1 H
L.A
A,C
A,C
B
CRC I
CRCSUM
H
CRCI
CRCSUM
H
B
PSW
CRC
PHP
PHA
STX
XTEMP
LDX LOX #0811 # 08>1
CRCI
I ASL
ADC AOC
A
#00H #OOH
STA
CRCTMP
LDA LOA
CRCSUM
ASL
A
STA
CRCSUM
LDA LOA CRCSUM+1
J
ROL
A
STA CRCSUM+1
CRCSUM+ J
ROL
A
EOR
CRCTMP
LSR
A
BCC
CRC2
LDA LOA CRCSUM+1
I
EOR #80H
STA
CRCSUM+ + II
LDA LOA
CRCSUM
EOR #5H #05H
STA STA
CRCSUM
CRC2 LDA LOA CRCTMP CRCTM P
DEX DEX
BNE CRCI
BNE
LDX LOX
PLA PLA
PLP
RTS
ENO
END
CRCI
XTEMP
XTEMP