haynes@DECWRL.DEC.COM (Charles Haynes) (08/05/86)
Stripping high order bits via the MOD operator is neither fast nor reliable. Unfortunately, modula-2 implementations differ on whether ORD(ch) can be negative or not, and how MOD operates on negative numbers. Secondly, MOD is usually equivalent to divide in cost, though many compilers optimize for DIV and MOD by powers of two. Mike Powell's modula-2 provides a module called "BitOperations" that will let you do bit AND, which would be fine, or in many cases an IF would be reasonable. IE. IF ORD(ch) >= 128 THEN ch := CHR(ORD(ch)-128); END; or IF ORD(ch) < 0 THEN ch := CHR(ORD(ch)+128); END; If you are willing to use machine dependent code, instead of reading into a buffer of chars, you could read into an array of things which looked like: BitChar = RECORD parity: BOOLEAN; ch: [0..127]; END; Then use the ch fields, this assumes your modula-2 compiler allows some kind of bit level control of record layout. Warning: this kind of code can often be slower than the naive "IF" statement used above. Know what you are doing! -- Charles
joel@decwrl.UUCP (08/05/86)
Yet another way to accomplish stripping off the parity bit is using set arithmetic. ch := char(bitset(ch) * bitset{0..6})) Of course, this may not generate great code either. Mike Powell's compiler for the VAX generates an extra mcoml instruction on the constant 127, rather than complementing at compile-time. (No AND instruction, just an AND NOT instruction on the VAX.) -- - Joel McCormack {ihnp4 decvax ucbvax allegra}!decwrl!joel joel@decwrl.arpa
treid@MITRE.ARPA (Thomas Reid) (08/05/86)
I mad thought to add to the bit stripping problem (nee, opportunity). The subject of machine dependence arose - a solution?: Put each possible machine interpretation in a case statement in a procedure in a module. Then use the initialization section of the module to test which of the possible cases are present for this execution and set the proper case selector (of course, a static variable inside the module). Overhead? The initialization is executed only once, so basically just the overhead of the case - order the case in order of likelihood and might be small. A clever idea what? Have the initialization test for machine dependence
ken@ROCHESTER.ARPA (Comfy chair) (08/05/86)
This is just an idea and probably should only be practiced by expert programmers under supervision :-), but how about using BITSETs and conversion operators? Something like: s := BITSET(ch) * {0,1,2,3,4,5,6}; ch := CHAR(s); Forgive me if I forgot the right syntax. Anyway this is also an excuse to try to weed out bad mail addresses with a test message. Thank you for your patience. I hope you enjoy the forum as much as I enjoy administering it. Cheers, Ken Info-Modula-2-request As always, meta-mail should go to info-modula-2-request.