ROMBox
                                 ======

The ROMBox provides an expandable method of adding ROMs to the Spectrum,
mapped in at &0000 to &3FFF, giving the following memory map:

FFFFh +-------+
      |       |
      |  RAM  |
      |       |
4000h +-------+-------+-------+------- - - -----+
      |       |       |       |                 |
      | ROM 0 | ROM 1 | ROM 2 | ROM 3 ... ROM 7 |
      |       |       |       |                 |
0000h +-------+-------+-------+------- - - -----+

ROM 0 is always present on Reset.  ROM 1 contains the standard Spectrum
ROM, usually provided by releasing ~ROMCS paging in the internal ROM. A
minimum ROMBox has one 16K ROM socket for ROM 0 and can page in ROM 0 or
ROM 1.

The ROM selection is controlled by writing to I/O port &FD:

        Bit   7   6   5   4   3   2   1   0
            +---------------+---------------+
            |   ROM Bank    |   RAM Bank    |   External Memory Select
            +---------------+---------------+

ROM Bank: Memory paged in at &0000    RAM Bank: Memory paged in at &C000

Bits 0 to 3 of the memory select latch are reserved for selecting banks of
RAM at &C000-&FFFF.

With a single-socket ROMBox, the following ROM bank values are available:

           %x0xx - Internal ROM(s)
           %0100 - External ROM
           %0101 - Internal ROM

The memory select latch is at almost the same address as the memory select
latch on a Spectrum 128.  The ROMBox responds to I/O address &xxFD and the
Spectrum 128 responds to I/O address %01xxxxxx:&FD.  The ROMBox is
designed to be selected using OUT (&FD),A instructions.

Selecting ROM %x0xx unpages ROMBox ROMs by releasing ~ROMCS and allows
internal ROMs to be accessed.  On the Spectrum 128 this will result in the
ROM selected by writing to &7FFD being accessed.

Selecting ROM %x1xx pages in ROMBox ROMs by asserting ~ROMCS. Selecting
ROM %0101 is defined as selecting the internal 48K Spectrum ROM.  On the
Spectrum 128 this results in port &7FFD being written to, selecting the
internal 48K Spectrum ROM.


Notes on OUT (&FD),A
--------------------
The OUT (nn),A instructions puts the value nn on the low byte of the
address bus and the contents of the A register on the high byte of the
address bus, and then writes the contents of the A register to the data
bus.  This effectively does OUT 256*A+nn,A.

The Spectrum 128 only decodes A15, A14 and Ax when decoding address &7FFD,
so executing an OUT (&FD),A instruction with A holding a value with b7=0
and b6=1 (ie &40 to &7F) will address the memory select latch.

The ROMBox takes advantage of this.  A program can write to both the
ROMBox select latch and the Spectrum 128 select latch by writing a value
in the range &40 to &7F to port &FD.  The ROMBox select latch can be
written to on its own by writing a value in the range &00 to &3F.

Of course, it is entirely possible to write to the ROMBox using BC
register OUT instructions. The ROMBox is designed to allow fast
single-register OUT instructions to access it.