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Text File | 1990-02-17 | 18KB | 473 lines

J M O D E M * The Microsoft C Version * February 17, 1990 Richard B. Johnson 405 Broughton Drive Beverly Massachusetts 01915 BBS (508) 922-3166 Introduction. JMODEM was first introduced about two years ago. It has en- joyed a steady increase in popularity around the world. It has even been explained in some detail in John Dvorak's book on PC communications; Dvorak's Guide to PC Telecommunica- tions, 1990, Osborne-McGraw-Hill, 2600 Tenth Street, Berkely, CA. JMODEM was first written in assembly language. Since this language is hard to read and use, it has been difficult for communications program developers to incorporate it into their programs directly. Instead it must be executed as an external protocol. Now JMODEM has been written in the C Language. C has become the de-facto standard for portable code development even though there isn't presently any code that is truly portable across many different machines. The writing of JMODEM in C will make it easier for software de- velopers throughout the world to use this very useful pro- tocol. You can use this new version of JMODEM just as the older versions written in assembly. It has a new color sign-on screen and status-block windows that overlap. Although the new JMODEM.EXE code is larger than the assembly-language version, JMODEM.COM, the program still executes very fast because much effort has been taken to streamline the C code. JMODEM.COM required 64 k of RAM (one segment) to execute properly. JMODEM.EXE requires 79 k to allow the screens to be written properly, and 66 k of free RAM to execute without aborting although the previous screen content will be lost. Like all versions of JMODEM, this version is 100% compatible with all previous version including Beta version 1.00. From it's inception, the essential structure of JMODEM has never been changed. How to install JMODEM. JMODEM executes best from a batch file as an external pro- tocol for any of the communications programs that have ex- ternal-protocol capability. A typical communications program is TELIX. - 1 - JMODEM, the C Version Here is a batch file used with TELIX for uploads: @ECHO OFF Rem * JMODEM TELIX Upload batch file. C:\TELIX\JMODEM S1 %3 Rem | || |_________ file name (passed by TELIX) Rem | ||____________ COM port (1 - 4) Rem | |_____________ Send Rem |_______________ Path and name of JMODEM This is a batch file used with TELIX for downloads: @ECHO OFF Rem * JMODEM TELIX Download batch file. C:\TELIX\JMODEM R1 %3 Rem | || |_________ file name (passed by TELIX) Rem | ||____________ COM port (1 - 4) Rem | |_____________ Receive Rem |_______________ Path and name of JMODEM This is a batch file used for PCPLUS (PROCOMM) uploads: @ECHO OFF Rem * JMODEM PCPLUS Upload batch file. C:\TELIX\JMODEM S1 %1 Rem | || |_________ file name (passed by PCPLUS) Rem | ||____________ COM port (1 - 4) Rem | |_____________ Send Rem |_______________ Path and name of JMODEM This is a batch file for PCPLUS (PROCOMM) ownloads: @ECHO OFF Rem * JMODEM PCPLUS Download batch file. C:\TELIX\JMODEM R1 %1 Rem | || |_________ file name (passed by PCPLUS) Rem | ||____________ COM port (1 - 4) Rem | |_____________ Receive Rem |_______________ Path and name of JMODEM You can chop this text out with an editor and create your batch files quickly and correctly. PCPLUS passes the file- name as the %1 parameter and TELIX uses the %3 parameter. JMODEM does not need to know anything else about your system to operate. It does not even need to know the baud-rate or other communications parameters. All it needs to know about is the file name and the communications adapter port to use. - 2 - JMODEM, the C Version JMODEM doesn't care if you are using a 19,200 baud modem with the DTE locked at high-speed or a 300 baud modem on a lamp-cord. It is very smart about communications and handles lost carriers (hangup) and flow-control in a very simple way which will be explained in detail later. If you only want to try JMODEM without having to install it, you need only to activate the JMODEM protocol on a BBS sys- tem, then "shell to DOS" and execute it manually from the DOS prompt. You will be pleasantly surprised at how rapidly it can transfer a file and how well it executes over net- works. How JMODEM works. JMODEM uses variable-length records called blocks. These blocks start with 512 data-bytes and increase in length to a maximum of 8192 bytes per block. There is a 6-byte overhead associated with each block so the percentage of overhead starts at a fairly high 0.1 percent and decreases to a very low 0.07 percent as the transmission progresses. The block length will increase in 512-byte increments as long as there are no errors requiring retransmission. Should an error occur, the block-size is cut in half. This continues until the block-size is as short as 64 bytes. The blocks may actually be of any length but never exceed the maximum allowed. An attempt is made to reduce the amount of data that needs to be transmitted by compressing each block before transmission. Since much data is already com- pressed, being from ".ZIP" files and such, it is possible that "compression" may actually cause an increase in block- length. JMODEM will send a compressed block only if it is shorter than the non-compressed block. Since it takes time to compress and expand data, only the simplest and quickest compression method is used. It is very effective with ".EXE" files that contain large blocks of binary nulls and most text files but it is not very effec- tive with highly compressed archive files. The compression method is called "run-length-limited" or more explicitly "how many of what kind". Basically, the block is searched for groups if similar characters. If many similar characters are found, compression consists of sending a sentinel byte of hexadecimal BB, followed by a two-byte amount, and this followed by the byte to be repeated. In the following example we have a string of spaces (20 hex). - 3 - JMODEM, the C Version Before compression: ( 18 bytes ) 20 20 20 20 20 20 20 20 20 20 20 34 37 87 EF FF 3A 23 After compression: ( 11 bytes ) BB 0B 00 20 34 37 87 EF FF 3A 23 The blocks could actually get longer because the sentinel byte could be present in the data: Before compression: ( 9 bytes ) BB 00 BB AF EF BB 00 AE EF After compression: ( 16 bytes ) BB 01 00 BB 00 BB 01 00 BB AF BB 01 00 BB AE EF As soon as the encoded length exceeds the data block length, compression is abandoned and the non-compressed block is sent. JMODEM sends a byte that tells the receiver if the data is compressed or not. This same byte tells the receiver when the end-of-file has occurred so JMODEM is able to preserve exact file-length. The JMODEM block. The JMODEM block looks like this: |< --------- First byte sent / received 20 00 01 01 .. .. .. .. .. AE 01 | | | | | | |___ CRC (high byte) | | | | | |______ CRC (low byte) | | | | |_______________ data bytes | | | |________________________ type of block | | |___________________________ block number | |______________________________ block length (high byte) |_________________________________ block length (low byte) The Block length: The block length is a WORD (16 bits). This allows the blocks to be 65,535 bytes long although in practice the length is not allowed to exceed 8192 bytes (plus overhead). The block- length is the length of the entire JMODEM block, not just the data-bytes. - 4 - JMODEM, the C Version The Block number: The block number is a BYTE. It starts at 1 and becomes zero again after block 255. It is used to make certain that all receiption errors are detected. The Block type: This BYTE is bit-mapped to tell the receiver what kind of block has been sent. Presently there are three kinds of blocks: 00000000B Hex 00 ( Normal data ) 00000001B Hex 01 ( Compressed data ) 00000010B Hex 02 ( End of file ) The CRC: The CRC is a WORD (16 bits). It is not a "standard" type of CRC because, unless done with hardware, a standard 16-bit polynoimial would take several seconds to calculate for a long block. Instead it uses a rapid rotate and sum algoritm that is probably just as effective as the more "standard" polynominials. The CRC is generated using this polynomial: X = X + X^(2(n-mod 7))....... Where n = t(n-1) And t = string length It has the advantage of simplicity in assembly-language programming and will detect errors with a probability of about one undetected error in 2^132 (which is a very large number). It does not correct errors so its not important to use some "standard" function to generate the CRC. The C code shows how easy it is to create and check this kind of CRC. - 5 - JMODEM, the C Version The JMODEM Communications Hardware Control When JMODEM is first loaded for execution it checks the state of the modem-control leads. If there is no modem carrier detected, it assumes that you have connected two PCs together without a modem and will not bother to check for a dropped carrier during execution. Since JMODEM must exercise flow-control so it can be used with high-speed modems with fixed baud rates, it also as- sumes that at the time at which it is first executed, the modem will be requesting data because there will have been very little I/O over the previous few seconds. JMODEM stores the state of RTS/CTS and DTR/DSR and uses this as a refer- ence. When transmitting data, should JMODEM find that the state of these modem-control leads has changed, it waits until the modem-control leads have reverted back to the initial state before sending any more data. This allows ANY modem to exercise flow-control with JMODEM, even those that use "pin 4" instead of "pin 20". When JMODEM is waiting, it checks to verify that the modem carrier has not been drop- ped. If the carrier is dropped, or if the user aborts, JMODEM will exit, setting a DOS ERRORLEVEL code. You can abort JMODEM at any time by hitting Ctrl-Break or Ctrl-C. It will take several seconds for JMODEM to abort because it does not continually check these keys. Unlike previous versions, JMODEM erases the files from aborted downloads. Also, JMODEM will abort if it is unable to rename a file in the following example. Suppose you wish to download a file called VIRUS.EXE. Sup- pose also that VIRUS.EXE already exists. Instead of over- writing your previous version of VIRUS.EXE, JMODEM has al- ways been nice and renamed it to VIRUS.OLD before creating a new file. With previous versions of JMODEM, if VIRUS.OLD already existed, JMODEM would have deleted it before renam- ing the present file to ".OLD". This no longer is done. JMODEM never deletes ANYTHING anymore except it's own abort- ed download. This should reduce the number of threats I have received! - 6 - JMODEM, the C Version The C programmer's Guide to JMODEM Included within this package should be the following files: JMODEM This is the MAKE file JMODEM_A C Contains the _main() routine entry point. JMODEM_B C Allocates memory, parses input strings. JMODEM_C C All of the file I/O is in this file. JMODEM_D C Data compression/expansion, CRC calculation JMODEM_E C Communications I/O interrupt service, etc. JMODEM_F C Screen I/O (windows, etc) JMODEM H Contains JMODEM globals and data structures. SCREEN H Screen definitions, prototypes, structures UART H Definitions for the 8250 UART If you have MicroSoft 'C' version 5.0 or later, and if your environment and paths are properly set up, you should be able to type: MAKE JMODEM ... and a brand new version of JMODEM will be created. Just as in the MASM language files, the source code is strongly-typed. All function prototypes are declared and no defaults are used. You can compile at warning-level 3, the most stringent level available with Microsoft compilers, and you will get no errors at all. You may modify JMODEM for your own use, but P L E A S E do NOT distribute the modified version on BBS systems because I do not wish to support 222,500 versions of JMODEM! If you find a bug, or wish to improve or add something that will increase the value of JMODEM without making it incompatible with previous versions, please upload your improvements to my BBS system and I may include the revisions (with your name attached) in an upcoming version. Note that there are 6 "type" bits available in the JMODEM control-byte that could be used to tell the receiver that another file is coming, etc., (for batch). Things like this could be added without destroying compatibility with previous versions. MicroSoft C seems to have several bugs one of which affects files that are very long (over 256k). My first attempts to use C files under MicroSoft for JMODEM used the stream-I/O (FILE *) type of files. This resulted in corruption of long files. Therefore I implemented the UNIX/DOS type of file-I/O that uses handles rather than file-control blocks. These work rather well. Another bug is in the SMALL C run-time library. Although you can compile the source to use the SMALL library version, and - 7 - JMODEM, the C Version the compiler / linker execute without errors, the resulting program will not work because the memory allocation routines dump the allocated buffers on top of each other! This re- sults in VERY corrupt files, bearing no resemblance to the originals! Stay with the medium models and you will have no problems. - 8 -