Gayle

In 1991 Commodore was designing a small, cheap Amiga, the A300, which reached the shops in 1992 as the A600. For it they needed a glue chip that did more than Gary: the new machine had an optional 2.5-inch IDE hard disk and a slot for PCMCIA cards, and neither existed in earlier Amigas. The answer was Gayle, an 84-pin gate array in a PLCC, and the specification says at once what it is for: running a 68000 Amiga with the ECS chipset at 7.16 MHz.

The list of functions has thirteen points. Decoding and timing for the ROM, for an optional flash ROM, for Chip RAM, for the chip registers, for the two CIAs, for the real-time clock, for the card connector, for the IDE disk and for an ARCNET network chip; generation of the CIAs' E clock; data buffer control; the system reset; and the floppy glue. Everything Gary did, and the rest.

The A1200, of 1992, carries a variant, the AA Gayle, for the AGA-chipset machine. The part numbers differ (391155 in the A600, 391424 in the A1200) and the chips are not swapped between the two machines.

The most visible novelty is IDE. Gayle produces the two chip selects an AT disk needs and the read and write signals with the right timing, and places the disk's registers at address $DA0000. There are two kinds of access: the control registers are read 8 bits wide, and the data register 16 bits wide, faster. The specification carries the register map just as it is in PCs, from the data register to the command register, with the disk's address bits wired to the processor's bits A2 to A4.

Today few A600s and A1200s in use have a mechanical disk: they carry a CompactFlash card or a memory module in its place, and it is through Gayle that they go.

The chip also provides for an optional flash ROM, at $A00000, meant to replace the floppy drive in an even cheaper version of the A300. As far as anyone knows it never came to be, but the address space stayed there.

The specification calls PCMCIA "credit card", the name the format had at the start. Gayle includes a complete interface to the card: 4 MB of memory at $400000, the card's attributes at $800000 and its I/O at $900000, plus four status and control registers. It supports, within limits, inserting and removing the card with the machine on, and can reset the system or raise a bus error when the card changes.

Two bits say a lot about the era: Gayle controls the 5 and 12 volt programming voltages that flash memory cards required, and two speed bits force 1.2 microsecond cycles for the card's memory and 560 nanosecond cycles for its I/O. It is through this slot that A600s and A1200s take network cards, CompactFlash and memory today.

The OVR signal lets a Zorro II card take the place of any of these spaces, the card's included: the specification's table lists what each zone normally is and what it becomes with OVR active.

Under the novelties is the old Gary. Gayle generates DTACK for every cycle, four clock cycles by default, and lets XRDY delay it and OVR switch it off. It decodes the ROM at $F80000 and $E00000, Chip RAM in the first 2 MB, the chip registers at $DFF000, the CIAs at $BFxxxx, and the real-time clock, a Ricoh RP5C01, at $DC0000.

The ROM overlay changed place: in Gayle, OVL is internal. It is active at reset, so the ROM appears at address zero, and switches off on the first write to CIA-B. It is a detail that matters on the bench: in an A600 or A1200, OVL is not measured on a CIA-A pin as in the A500.

The reset comes from the keyboard: _KBRESET pulls the processor's reset and halt outputs, and an external reset clears all of Gayle's states. And the floppy glue is the same: the motor latched when the drive is selected, and the write signals buffered, all open collector.