Section 2 - Commodore Computers

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11A 114 COMPUTE! Mov,W82.lssue May, 1982. Issue 24 counting for one error in a transfer of 10,240 bytes, the transfer time would be about six minutes. I If f the records were 256 bytes long, the transfer time would be about 5.9 minutes, and with 1024 byte records it would be about 6.5 minutes. It would seem that the 256 byte format would be the best choice, but another factor must be taken into account: the error detection method used. In the method used for the CP/M system, it is very simple, and the more bytes it is required to check, the greater the chance is that it will miss an error. r. Because of this, the 128 byte format is a better choice even though there is a small increase in m the transfer time. Ifa a beller better error detection method were to be used, it would probably be beller better to use the 256 byte record format. A Transfer Format The actual structure re of the record that is transfen'ed ferred varies from system to system as well. In [n fact, it is even less standardized than most other pans parts of the data transfer format. Since there is no' real standard forr the record format, I will describe one of the more heavily used formats. This s format got its start on CP/M· CP/M- based systems and originally appeared in a program written by Ward Christensen called, appropriately enough, "MODEM." The first problem when dealingg with CP/M is its refusal to acknowledge the existence of a modem. So a transfer program must provide its own link to the modem. To begin the transfer, the receiving computer sends an ASCII NAK K (15H) signal every couple of seconds until unlilthe sending computer sends an ASCII ACK (06H). This is the synchronization part of the transfer. TheT original modem program assumed that the program was predefined at both ends, so, once synchronization ni was achieved, the data was immediately sent. The record format that is used consists of a header, the data, and finally y a checksum character for error detection. The header consists of an ASCII SOH character (OIH) (0 I followed by the current record number (starting with number one) o which is an eight bit value. That is followed owed by the same number, but inverted. (That is, if record 01H0 I H is being sent, then the second number sent will be FEH.) This is then followed by the data itself for the next 128 bytes. Finally, one more character is sent which is the checksum. The checksum is an eight bit value e that is the sum m (without carry) of alla the data bytes sent. The sending computer compuLer then waits for the receiving recei\' in g computer to acknowledge that it received the e data. daLa. The l ~ h e receiving g computer compares cOlllpares its own calculated checksum against the one that Lhat the one that Lhat the sending computer sent and, if they match, it sends an ASCII ACK character (06H). If they don't match, it sends an ASCII N NAKA K character ((15H), indicating that it il didn't receive the data correcLl y. IfLhesending compute r receives a NA K. correctly. If the sending computer receives a NAK, it will send the record again. . I If, r. afLer after Len ten tries it is unable to send the record it gives up and aborts the unable to send the record it gives up and aborts the transfer. After all off the recordss have been sent, a final ASC II EOT (04 H) is sent indicating that the transmissio n is completed. final ASCII EOT (04H) is sent indicating that the transmission is completed. Some Problems There are several problems wi th the format that is There are several problems with the format that is used and some of the later versions attempted to correct for this. Unfortunately, this created a new problem since any change in the basic format meant it was incompatible with the old format. This tended to lead towards a real mess with patches to allow for compatibility to the old programs. Discounting the versions which were simply adaptations for the versions wh ich were simply adaptations for different modems, some of the differences that to specify it. T here was also change from the check different modems, some of the differences that have occurred are the addition of the program identifier so that the sending computer cann tell the receiving computer the progl'am program name instead of requiring the operator r at the receiving compuLer computer to specify it. There was also change from the check sum format to a CRC format. The identifier has been implemented several ways, but bUL the most popular version is also one of the sLrangesL strangest i implementations. III Lations. A After I' LeI' synchronization has been achieved, , the currently popular program (MODEM7) (MODEM?) sends the filename e nanle a character at a titlle time. . That is, it sends a character and then wails waits for an acknowledge (ASCII ACK) from the receiving g computer. compuLer. Then it iL sends the next character of the filename and repeats this until the entire filename has been sent. After AfLer that, thaL, it iL waits for the receiving computer to send the checksum of the filename and then Lhen compares the received checksum against againsL its iLs own inter nally calculated checksum. If they are equal, the nall y calculaLed checksum. If Lhey are equal, the sending computer se nds an ASC I [ ACK character. sending computer sends an ASCII ACK character. The sending computer puLer goes back and waits wailS for resynchronization. (Waiting (' Na iLing for i'or an ASCII NAK character.) Ler.) At AL last, lasL, it iL starts sLarts receiving g data normally y after the resynchronization ni zalion is achieved. If there was a checksum error, the sending If there was a checksum error, the sending computer sends a bad name character characLer which, for no particular panicular reason, , was defined as an ASCII // /I (75H) 1-1 ) and goes back to LU allow resynchronization ynchronizaLion and retransmission rCLranslllission of the name. If, I f. after ten len tries the name cannot not be sent, the transfer is aborted. Improved Error Detection Another AnoLher problem that Lhat some versions have corrected correcLed for fa r is that thaL the checksum method meLhod of error detection deLection is not t the e most accurate means ofo f detecting an error. A CRC (cyclic ic redundancy check) is a far
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Section 2 - Commodore Computers

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