Jun 15, 2003 15:21 from Scalar I believe one of the new opcodes offered by the 65C02 is known as "BRA", or "branch always".
Normally a branch operates as a test of something else, "branch to this location if this condition is met". Branch Always will branch without any tests. The main use for it is the memory portability I mentioned, allowing a program to be relocated in memory and still work properly without remapping the JMP's. A sufficiently clever programmer can jump long distances through a program by using multiples BRA's to skip like a rock through the code. The downside is that this is slow, and it consumes memory space. 2000: (want to get back here from end) .... 207C: BRA 2080 (hop over next command and continue forward) 207E: BRA 2000 (go back 126 bytes) 2080: (program continues forward) .... 20FA: BRA 20FE (hop over next command and continue forward) 20FC: BRA 207E (go back 126 bytes) 20FE: (program continues forward) .... 2179: BRA 217C (hop over next command and continue forward) 217A: BRA 20FC (go back 126 bytes) 217C: (program continues forward) .... (etc) Using the BRA's like this, the program is fully portable in memory, and will run fine from any memory location, whether $0900 or at $6000.. :) [Antique And Older Computers> msg #24010 Steve Conrad 810 Main Henrietta, MO 64036 816-494-5692 http://sasha91.0pi.com www.fortunecity.com/rivendell/stonekeep/600 It is no secret (nor should it come as any surprise) that humankind's most noble impulses often surface during the most trying of times, that human spirit rises to the challenge when faced with adversity, that human strength is born from human failings...Is it any wonder, then, that the SDF-1 crew became a tighter family after the fortress had been exiled than it had before? From the log of Captain (later Admiral) Henry Gloval -- Apple2list is sponsored by <http://lowendmac.com/> and... / Buy books, CDs, videos, and more from Amazon.com \ / <http://www.amazon.com/exec/obidos/redirect-home/lowendmac> \ Support Low End Mac <http://lowendmac.com/lists/support.html> Apple2list info: <http://lowendmac.com/lists/apple2.html> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/apple2list%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
