THE +2A/+3 SPECTRUMS

The +2A/+3 share the same timing information, sound chip, etc as the
128K/+2 machines; see above for details.

Bit 6 of port #FE of the +2A/+3 does not show the same dependence on what
was written to port #FE as it does on the other machines, and always
returns 0 if there is no signal. Finally, reading from a non-existing port
(eg #FF) will always return 255, and not give any screen/attribute bytes
as it does on the 48K/128K/+2.

                                   Memory

The basic principle of paging on the +2A and +3 is the same as for the
128K/+2. However, the +2A and +3 have four ROMs rather than two, and
certain extra memory configurations.

Port 7FFDh behaves in the same way as before, except that bit 4 is now the
low bit of the ROM selection. The extra features are controlled by port
1FFDh. This port is also write-only, and its last value should be saved at
5B67h (23399).

Port 1FFDh responds thus:

  Bit 0: Paging mode. 0=normal, 1=special
  Bit 1: In normal mode, ignored.
  Bit 2: In normal mode, high bit of ROM selection. The four ROMs are:
          ROM 0: 128k editor, menu system and self-test program
          ROM 1: 128k syntax checker
          ROM 2: +3DOS
          ROM 3: 48 BASIC
  Bit 3: Disk motor; 1=on, 0=off
  Bit 4: Printer port strobe.

When special mode is selected, the memory map changes to one of four
configurations specified in bits 1 and 2 of port 1FFDh:

       Bit 2 =0    Bit 2 =0    Bit 2 =1    Bit 2 =1
       Bit 1 =0    Bit 1 =1    Bit 1 =0    Bit 1 =1
 FFFFh+--------+  +--------+  +--------+  +--------+
      | Bank 3 |  | Bank 7 |  | Bank 3 |  | Bank 3 |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
      |        |  | screen |  |        |  |        |
 C000h+--------+  +--------+  +--------+  +--------+
      | Bank 2 |  | Bank 6 |  | Bank 6 |  | Bank 6 |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
 8000h+--------+  +--------+  +--------+  +--------+
      | Bank 1 |  | Bank 5 |  | Bank 5 |  | Bank 7 |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
      |        |  | screen |  | screen |  | screen |
 4000h+--------+  +--------+  +--------+  +--------+
      | Bank 0 |  | Bank 4 |  | Bank 4 |  | Bank 4 |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
      |        |  |        |  |        |  |        |
 0000h+--------+  +--------+  +--------+  +--------+

RAM banks 1,3,4 and 6 are used for the disc cache and RAMdisc, while Bank
7 contains editor scratchpads and +3DOS workspace.

The contended memory timings differ on the +2A/+3 from the earlier
machines; firstly, the timing differences mean that the top-left pixel of
the screen is displayed 14364 T-states after the 50 Hz interrupt occurs,
as opposed to 14336. The T-states (relative to the interrupt) at which
delays occur are given in the following table:

      Cycle #    Delay
      -------    -----
       14361       1
       14362   No delay
       14363       7
       14364       6
       14365       5
       14366       4
       14367       3
       14368       2
       14369       1
       14370   No delay
       14371       7
       14372       6

and so on, until cycle 14490, when the display of the first scanline on
the screen has been completed, and no more delays are inserted until 14589
(=14361+228) when the cycle repeats. The other difference occurs for
instructions which have multiple 'pc+1' or 'hl' entries in the breakdown
for the other machines: on the +2A/+3, these entries are combined into
just one. This means that, for example, JR becomes pc:4,pc+1:8.

Like the base 128K machine, RAM banks 4-7 are contended. However, port #FE
is not; whether ports #7FFD and #1FFD are contended is currently unknown.

                                 Disk Drive

Another useful bit of information is that it is possible to add an Amiga
3.5" drive to a Spectrum +3 by Simply wiring the pins by name and
providing it with some external power source (which can be an Amiga PSU
with increased amperage, used to power the entire +3 and external drive in
one go). There is a public domain formatter around to format it to 720k -
this is quite useful as 3" disks are now a rarity and were never that
reliable.

The added advantage is that PC users equipped with a copy of Sybex's
shareware package 22Disk can read these files into a PC too.

                                  [Image]