Another Panasonic HHC

Well, the internal ROMs -- the user-installable ROMs -- are bank-switched too, I think. There are 3 outputs from the I/O controller (custom chip). One of them selects between {a} all acessses to a particular area going to the I/O devices and {b} write accesses going to the I/O stuff and reads going to the ROMs. The other 2 lines are decoded by half a '139 to select between the ROMs (3 internal sockets, one signal on the expansion connector for one (or is it several, with external bank select logic) ROM in an add-on unit. But I would be suprised if the SNAP development ROMs would work without changes in the internal sockets. Also on the HHC, I've discovered that the display really is 159 columns of dots. There are 4 driver chips, each of them can drive 40 columns. The 160th 'column' is the 8 annunciators across the bottom of the display. The 6502 instruction set is so small that it's easy to write a disassembler... -tony