In article <803437094.AA01156@clone.his.com>, Matthew Russotto wrote: > > The proms were P5 and P6 (13 sector -- 16 sector had P5A and P6A). But both > were connected to the bus, they were 256*4 PROMs. Wrong. P5/P5A and P6/P6A are both "6309" chips, and are 256x8 PROMs. They are 20 pins, with 8-bit address, 8-bit data, power, group, and two active-low enable inputs. P5/P5A is the slot PROM ($Cn00-$CnFF). P6/P6A is the "logic state sequencer" PROM, which is used to control disk accesses. Its contents are not directly accessible by the Apple II. There is a circuit diagram of the Disk ][ controller in the DOS 3.2 Reference Manual, along with the Disk ]['s analog board. These diagrams were not included in later manuals. > The Disk II controller didn't have a processor, so any ROM not > connected to the 6502 would be quite useless. Also wrong. The P6 PROM is combined with a 74LS323 universal shift register, 74LS174 6-way D-flip-flop and 74LS259 addressable latch to form a very simple "processor", which implements the very low-level operations involved in the disk access, such as shifting individual bytes in and out. State machine actions are initiated by the Apple II accessing various combinations of the "Q6" and "Q7" soft-switches (the two high-order bits of the 74LS259), as well as the previous contents of the state sequencer ROM and various other inputs. The operation of the state machine is documented briefly in "Beneath Apple DOS" and "Beneath Apple ProDOS", and in more detail in "Understanding the Apple II" (or IIe), all from Quality Software, now stocked by Byte Works. -- David Empson dempson@actrix.gen.nz Snail mail: P.O. Box 27-103, Wellington, New Zealand