Special banking mode of the 128K +2A/+3 ZX-Spectrum:
The $1FFD register (standard on the Amstrad 128K +2A/+3 ZX-Spectrum) is supported and, when active, completely redefines the whole memory map as follows:
Bit 0: Paging mode. 0=normal, 1=special
Bit 1: In normal mode, ignored.
Bit 2: In normal mode, high bit of ROM selection (A15)
When special mode is selected, the memory map changes to one of four configurations specified in bits 1 and 2 of port $1FFD:
Bit 2 =0 Bit 2 =0 Bit 2 =1 Bit 2 =1
Bit 1 =0 Bit 1 =1 Bit 1 =0 Bit 1 =1
0xffff +--------+ +--------+ +--------+ +--------+
| Bank 3 | | Bank 7 | | Bank 3 | | Bank 3 |
| | | | | | | |
| | | | | | | |
| | | screen | | | | |
0xc000 +--------+ +--------+ +--------+ +--------+
| Bank 2 | | Bank 6 | | Bank 6 | | Bank 6 |
| | | | | | | |
| | | | | | | |
| | | | | | | |
0x8000 +--------+ +--------+ +--------+ +--------+
| Bank 1 | | Bank 5 | | Bank 5 | | Bank 7 |
| | | | | | | |
| | | | | | | |
| | | screen | | screen | | screen |
0x4000 +--------+ +--------+ +--------+ +--------+
| Bank 0 | | Bank 4 | | Bank 4 | | Bank 4 |
| | | | | | | |
| | | | | | | |
| | | | | | | |
0x0000 +--------+ +--------+ +--------+ +--------+
ROM is never accessed when a special mode is active. This register is write-protected when D5 of $7FFD register is SET, as in the original hardware.
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