Memory Mapping Mode Register

Number	TBRegisterNumber::$8F
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::Select memory mapping mode

 bits 7:2 = Reserved, must be zero
 bits 1:0 = Mapping mode applied (hard reset = 0)
   00 = Standard ZX 128k +3
   01 = Reserved
   10 = Pentagon 512K
   11 = Pentagon 1024K (limited to 768K on 1MB machines)

 * Standard ZX 128K +3 = principally ports 0x7ffd, 0xdffd, 0x1ffd
 * Pentagon 512K = principally port 0x7ffd
 * Pentagon 1024K = principally ports 0x7ffd, 0xeff7
 ** The mapping modes affect how ports 0x7ffd, 0xdffd, 0x1ffd and 0xeff7 carry out memory paging, see ports.txt