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.Appendix IV gives a list of the full ASCIIimplementation and the usual 'special' codes as they apply tocomputer-to-computer communications.File Protocols - When computers are sending large files to eachother, a further layer of protocol, beyond that defining individualletters, is necessary.For example, if your computer is automaticallyfile:///E|/Books/Hackers Handbook.htm (24 of 133) [11/28/2000 5:58:48 AM] Hacker's Handbooksaving to disk at regular intervals as the buffer fills up, it isnecessary to be able to tell the host to stop sending for a period,until the save is complete.On older time-share services, where thetypical terminal is a teletypewriter, the terminal is in constantdanger of being unable mechanically to keep up with the hostcomputer's output.For this reason, many host computers use one oftwo well-known protocols which require the regular exchange ofspecial control characters for host and user to tell each other allis well.The two protocols are:Stop/Start - The receiving computer can at any time send to the hosta Stop (ctrl-S) signal, followed by, when it is ready a Start,(ctrl-Q).EOB/ACK - The sending computer divides its file into a blocks (of anyconvenient length); after each block is sent, an EOB (End of Block)character is sent (see ASCII table, Appendix IV).The user's computermust then respond with a ACK (Acknowledge) character.These protocols can be used individually, together or not at all.You may be able to use the 'Show Control Codes' option to checkwhether either of the protocols are in use.Alternatively, if youhave hooked on to a service which for no apparent reason, seems tostop in its tracks, you could try ending an ACK or Start (ctrl-F orctrl-S) and see if you can get things moving.File transmission - All terminal emulators assume you will want tosend, as well as receive, text files.Thus, in addition to theprotocol settings already mentioned, there may be additional ones forthat purpose, e.g.the XMODEM protocol very popular on bulletinboards.Hackers, of course, usually don't want to place files onremote computers.Specific terminal emulation - Some software has pre-formatted sets ofcharacteristics to mimic popular commercial 'dumb' terminals.Forexample, with a ROM costing under £60 fitted to a BBC micro, you canobtain almost all of the features of DEC's VT100 terminal, whichuntil recently was regarded as something of an industry-standard andcosting just under £1000.** Page 24Other popular terminals are the VT52 and some Tektronix models, thelatter for graphics display.ANSI have produced a 'standard'specification.Baudot characters - The Baudot code, or International TelegraphicCode No 2, is the 5-bit code used in telex and telegraphy -- and inmany wire-based news services.A few terminal emulators include it asan option, and it is useful if you are attempting to hack suchservices.Most software intended for use on radio link-ups (seeChapter 10) operates primarily in Baudot, with ASCII as an option.Viewdata emulation - This gives you the full, or almost full,graphics and text characters of UK-standard viewdata.Viewdata tvfile:///E|/Books/Hackers Handbook.htm (25 of 133) [11/28/2000 5:58:48 AM] Hacker's Handbooksets and adapters use a special character-generator chip and a few,mostly British-manufactured, micros use that chip also-- the AcornAtom was one example.The BBC has a teletext mode which adopts thesame display.But for most micros, viewdata emulation is a matter ofusing hi-res graphics to mimic the qualities of the real thing, or tostrip out most of the graphics.Viewdata works on a screen 40characters by 24 rows, and as some popular home micros have 'native'displays smaller than that, some considerable fiddling is necessaryto get them to handle viewdata at all.In some emulators, the option is referred to as Prestel orMicronet--they are all the same thing.Micronet-type software usuallyhas additional facilities for fetching down telesoftware programs(see Chapter 10) [ Pobierz caÅ‚ość w formacie PDF ] |
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