Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cs.utexas.edu!sun-barr!cronkite!newstop!sun!imagen!atari!apratt From: apratt@atari.UUCP (Allan Pratt) Newsgroups: comp.sys.atari.st.tech Subject: Re: Mega STe Questions Message-ID: <2968@atari.UUCP> Date: 21 Jun 91 16:47:58 GMT References: <1991Jun19.114741.11944@informatik.uni-erlangen.de> <1991Jun20.122304.23243@ugle.unit.no> Distribution: comp Organization: Atari Corp., Sunnyvale CA Lines: 36 >In article <1991Jun19.114741.11944@informatik.uni-erlangen.de>, >csbrod@immd4.informatik.uni-erlangen.de (Claus Brod) writes: >> The 68881 is optional. The Mega STE's FPU is programmed just like >> the FPUs available for the standard Mega, so there should be some >> development packages supporting it. This is correct. stigvi@Lise.Unit.NO (Stig Vidar Hovland) writes: >How is it programmed on a standard Mega? Will programs written for the >TT and 68882 run correctly on a Mega STE and 68881? No. The 68030 communicates with the FPU as a coprocessor. You use instructions like "FSIN fp0,fp1" and the CPU communicates directly with the coprocessor, telling it to take the sine of FP0 and put the answer in FP1. (FP0 and FP1 are floating-point data registers in the FPU.) The 68000 does not have this ability. With a 68000 the FPU is a peripheral, like the sound chip or floppy-disk controller: it takes many regular 68000 instructions to read and write I/O ports on the chip to get it to do what you want. You have to do it "by hand" instead of having the CPU do it for you. Thus, it is slower and your program is larger than it would be using 68030 FPU instructions. What Claus is saying is that there is an FPU peripheral that plugs into the Mega ST (called the SFP004), and there is an FPU that comes with the Mega STe, and they interface the same way. They both interface in the "peripheral" way, because that's the only way the 68000 can handle. This means that a program (or library) that runs on a Mega using the SFP004 will run on a Mega STe and use its built-in FPU. In the TT the FPU interfaces in the "coprocessor" way, so these programs won't work. ============================================ Opinions expressed above do not necessarily -- Allan Pratt, Atari Corp. reflect those of Atari Corp. or anyone else. ...ames!atari!apratt