Front Panels - Thoughts

If you have a machine that allows for other devices to become a bus master and access memory and I/O devices (S100 does this) then it is relatively easy to make a front panel that does just that. You can then use said panel to access memory, but not, of course proceessor registers. Somebody mentioned the PCjr. Unlike the PC (and all its descendants), the PCjr doesn't have a internal DMA controller but allows for one to be added on the expansion port. As a result, a device there can take over the system bus, just what you need for a front panel. For the machines I mentioned earlier (PDP11s, VAXen, PERQs), the Unibus does of course allow for multiple bus masters. But in the case of the 11/44 and VAX11/730 main memory is not on the Unibus (which only allows for 18 bit addressing), there is an address translator which is configured by the CPU between Unibus addresses and main memory addresses. So a DMA peripheral doesn't get access to all of memory (unlilke on smaller PDP11s). And of course you wouldn't get access to processor registers, etc that way. And the PERQ doesn't allow for other bus masters at all. Memory control on that machine was designed in conjunction with the I/O system (which includes DMA controllers and the graphics system). -tony