Next's memory can be paged into the top 16K. Also in this mode, **D3** of port **EFF7h** can be set to place 16K bank 0 into the bottom 16K, overlaying the ROM. Writes to ports **7FFDh**, **DFFDh**, **1FFDh** and **EFF7h**, will cause the bottom 16K and the top 16K to remap; if allRAM mode is selected, the entire 64K remaps. In Pentagon 512K mode, port **7FFDh** is extended so that bits **D7:D6** augment the bank number in bits **D2:D0** to form a 5-bit bank number for the top 16K. On the ZX Spectrum Next, port **1FFDh** continues to function (allRAM mode and ROM selection) and **D3** of port **EFF7h** can be set to map 16K bank 0 on top of ROM in the bottom 16K. Writes to ports **7FFDh**, **DFFDh**, **1FFDh** and **EFF7h** will cause the bottom 16K and the top 16K to remap; if allRAM mode is selected, the entire 64K remaps. In Pentagon 1024K mode, port **EFF7h D2** = **0** enables Pentagon 1024K mapping and port **EFF7h D2** = **1** selects standard ZX Spectrum mapping as described above. With Pentagon 1024K mode active, port **7FFDh** is unlocked and **D5** (the lock bit) is repurposed as another bank bit. The bank number paged into the top 16K is taken as port **7FFDh** bits {**D5,D7,D6,D2,D1,D0**} to reach 1MB in 16K banks. On the Next, port **1FFDh** continues to function (allRAM mode and ROM selection) and **D3** of port **EFF7h** can be set to map 16K bank 0 on top of ROM in the bottom 16K. Writes to ports **7FFDh**, **DFFDh**, **1FFDh** and **EFF7h** will cause the bottom 16K and the top 16K to remap; if allRAM mode is selected, the entire 64k remaps. For 100% compatibility with the original banking methods listed above, extra ports like **1FFDh** can be disabled using the internal port decodes with **NextREGs 130 - 133** (**82h - 85h**). ### The Expansion Bus The Expansion Bus is found in the back of the ZX Spectrum Next and exposes its CPU to the world. As it too gets addressed by **IN** and **OUT** commands, it is listed below:

Bottom Side	A11	28	28	Reserved	Top Side
	A9	27	27	A10
	B̅U̅S̅A̅C̅K̅	26	26	A8
	R̅O̅M̅C̅S̅	25	25	R̅F̅S̅H̅
	A4	24	24	M̅1̅
	A5	23	23	NC
	A6	22	22	NC
	A7	21	21	W̅A̅I̅T̅
	R̅E̅S̅E̅T̅	20	20	NC
	BUSREQ[^p246-6]	19	19	W̅R̅
	NC	18	18	R̅D̅
	NC	17	17	I̅O̅R̅Q̅
	Reserved	16	16	M̅R̅E̅Q̅
	R̅O̅M̅C̅S̅	15	15	H̅A̅L̅T̅
	GND	14	14	N̅M̅I̅
	I̅O̅R̅Q̅U̅L̅A̅	13	13	I̅N̅T̅
	A3	12	12	D4
	A2	11	11	D3
	A1	10	10	D5
	A0	9	9	D6
	C̅L̅K̅	8	8	D2
	GND	7	7	D1
	GND	6	6	D0
	Key	5	5	Key
	+9V (PSU)[^p246-7]	4	4	R̅O̅M̅C̅S̅
	+5V	3	3	D7
	A12	2	2	A13
	A14	1	1	A15

*The ZX Spectrum Next Expansion Bus* [^p246-6]: *BUSREQ is incorrectly active High oin Issue 2 but fixed on Issue 4* [^p246-7]: *This pin receives the unregulated power from the PSU line. If you plug a higher voltage PSU, that voltage will be present at that pin and may damage your peripherals*