Doug McNutt wrote: > The 68 k series had instructions that, if not present in the hardware, would > generate "A_line" traps that would interrupt to run user-provided code buried > somewhere in the operating system. > > I doubt that the OS did much to emulate a real PMMU but it's quite possible > that Apple's virtual memory did. The floating point instructions were > certainly emulated by A_line traps. >
If the CPU attempted to execute any non-existant instruction an instruction exception occured. All the AXXX and FXXX instructions were un-implemented. FXXX instructions were reserved for floating point operations and are implemented in the 68881 and the 68040 (full version). The story is Motorola reserved the AXXX instructions for something but never used them since Apple usurped them. AXXX instrucions (A line traps) were used to make OS calls, so there was room for 4096 OS calls. Floating point emulation was done using F line traps as they were reserved for floating point instructions. That way the code was the same whether or not there was an FPU. -- Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting "I'm a designated driver on the Information Super Highway" -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
