Page 236 | ZX Spectrum Next User Manual, 3rd Edition

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Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Coordinate (X_beg,X_end,Y_beg,Y_end)	■	■	•	•	•	•	•	•	•	•	8-bit value X_beg,X_end,Y_beg,Y_end coordinate¹		*

1 1^st Write X_beg position – *Resets to 0
2^nd Write X_end position – *Resets to 255
3^rd Write Y_beg position – *Resets to 0
4^rh Write Y_end position – *Resets to 191
Reads do not advance the clip position – Use NR 28 (1Ch) D2:D3 to read the position. If need be, write to NR 28 (1Ch):D1 to reset the clip index. When the clip window is enabled for sprites in over border mode, the X coordinates are internally doubled and the clip window origin is moved to the sprite origin inside the border

NextREG 26 (1Ah) – Layer 0 (ULA/LoRes) Clip Window Definition

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Coordinate (X_beg,X_end,Y_beg,Y_end)	■	■	•	•	•	•	•	•	•	•	8-bit value X_beg,X_end,Y_beg,Y_end coordinate¹		*

1 1^st Write X_beg position – *Resets to 0
2^nd Write X_end position – *Resets to 255
3^rd Write Y_beg position – *Resets to 0
4^rh Write Y_end position – *Resets to 191
Reads do not advance the clip position – Use NR 28 (1Ch) D4:D5 to read the position. If need be, write to NR 28 (1Ch):D2 to reset the clip index
NOTE: LoRes may get a separate clip window in the future

NextREG 27 (1Bh) – Layer 3 (Tilemap) Clip Window Definition

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Coordinate (X_beg,X_end,Y_beg,Y_end)	■	■	•	•	•	•	•	•	•	•	8-bit value X_beg,X_end,Y_beg,Y_end coordinate¹		*

¹ 1^st Write X_beg position – *Resets to 0
2^nd Write X_end position – *Resets to 159
3^rd Write Y_beg position – *Resets to 0
4^rh Write Y_end position – *Resets to 255
Reads do not advance the clip position – NR 28 (1Ch) D6:D7 reads the position. If need be, write to NR 28 (1Ch):D3 to reset the clip index and then do consecutive writes and reads to get to the value you're searching for. The X coordinates are internally doubled

NextREG 28 (1Ch) – Clip Windows Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Layer 2 Clip Index	■		colour: pink	colour: pink	colour: pink	colour: pink	colour: pink	colour: pink	•	•	Layer 2 Clip Index (0 – 3)
Sprites Layer Clip Index	■		colour: pink	colour: pink	colour: pink	colour: pink	•	•	colour: pink	colour: pink	Sprites Clip Index (0 – 3)
Layer 0 / 1 Clip Index	■		colour: pink	colour: pink	•	•	colour: pink	colour: pink	colour: pink	colour: pink	ULA / Timex Clip Index (0 – 3)
Layer 3 Clip Index	■		•	•	colour: pink	colour: pink	colour: pink	colour: pink	colour: pink	colour: pink	Tilemap Clip Index (0 – 3)
Reserved		■	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	colour: pink	colour: pink	colour: pink	Reserved, must be 0
Layer 2 Clip Index Reset enable		■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	colour: pink	colour: pink	•	1 to Reset the Layer 2 Clip Index
Sprites Clip Index Reset enable		■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	colour: pink	•	colour: pink	1 to Reset the Sprites Clip Index
Layer 0/1 Clip Idx Reset enable		■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	•	colour: pink	colour: pink	1 to Reset the ULA/Timex Clip Index
Layer 3 Clip Idx Reset enable		■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	colour: pink	colour: pink	colour: pink	1 to Reset the Tilemap Clip Index

NOTE: This NextREG may change in the future

NextREG 30 (1Eh) – Active Video Line MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Active Video Line (MSB)	■		colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	1 for lines above 255 else 0
Reserved	■		colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	Reserved, must be 0

NextREG 31 (1Fh) – Active Video Line LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Active Video Line (LSB)	■		•	•	•	•	•	•	•	•	Active Video Line (LSB)

NextREG 32 (20h) – Maskable Interrupt Generation

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
CTC	■	■	colour: pink	colour: pink	colour: dark green	colour: dark green	0	0	0	0	No CTC interrupt generated
			colour: pink	colour: pink	colour: dark green	colour: dark green	colour: pink	colour: pink	colour: pink	•	Generate CTC 0 interrupt
			colour: pink	colour: pink	colour: dark green	colour: dark green	colour: pink	colour: pink	•	colour: pink	Generate CTC 1 interrupt
			colour: pink	colour: pink	colour: dark green	colour: dark green	colour: pink	•	colour: pink	colour: pink	Generate CTC 2 interrupt
			colour: pink	colour: pink	colour: dark green	colour: dark green	•	colour: pink	colour: pink	colour: pink	Generate CTC 3 interrupt
Reserved	■	■	colour: pink	colour: pink	colour: dark green 0	colour: dark green 0	colour: pink	colour: pink	colour: pink	colour: pink	Reserved, must be 0
ULA	■	■	colour: pink	•	colour: dark green	colour: dark green	colour: pink	colour: pink	colour: pink	colour: pink	Generate ULA interrupt
Line	■	■	•	colour: pink	colour: dark green	colour: dark green	colour: pink	colour: pink	colour: pink	colour: pink	Generate Line interrupt

* Set bits on R indicate whether an interrupt occurred or is pending – alias of bits in NextREGs 200 - 202 (C8h - CAh)
* Set bits on W always generate a maskable interrupt ignoring enables – NextREGs 196 - 198 (C4h - C6h)

NextREG 34 (22h) – Line Interrupt Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Line Interrupt Value MSB	■	■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	colour: pink	•	MSB of line number		0
Line Interrupt Control	■	■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	•	colour: pink	Enable Line Interrupt*		0
ULA Interrupt Control	■	■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	colour: pink	colour: pink	Disable ULA Interrupt*		0
Reserved		■	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	colour: pink	colour: pink	Reserved, must be 0
Reserved	■		colour: light green	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	colour: pink	colour: pink	Reserved, must be 0
ULA Interrupt Signal	■		•	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	colour: pink	colour: pink	ULA is currently asserting interrupt

* Aliases of interrupt enable bits in NextREG 196 (C4h)

NextREG 35 (23h) – Line Interrupt Value LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Line Interrupt Value LSB	■	■	•	•	•	•	•	•	•	•	Lower 8-bits of line number		0

NextREG 36 (24h) – Reserved

Protection against OUT 3Bh, A – See DISCiPLE disk interface for example.
In legacy modes set selected NextREG to 36 (24h)

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Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
X Offset	■	■	•	•	•	•	•	•	•	•	8-bit value of X Offset (0-255)		0

This setting refers to all ULA modes except Layer 1,0 – LoRes
NextREG 104 (68h):D2 adds a half pixel to the scroll

NextREG 39 (27h) – ULA Vertical Scroll Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Y Offset	■	■	•	•	•	•	•	•	•	•	8-bit value of Y Offset (0-191)		0

This setting refers to all ULA modes except Layer 1,0 – LoRes

NextREG 40 (28h) – Stored Palette Value and PS/2 Keymap Address MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Stored Palette value	■		•	•	•	•	•	•	•	•	See NextREG 68 (44h)
PS/2 Keymap Address MSB		■	colour: pink	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	MSB of value in NR 41 (29h)
Keymap Selection		■	0	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	Select PS/2 Keymap		0
Keymap Selection		■	1	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: pink	Select key joystick		0
Reserved		■	colour: pink	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	Reserved, must be 0

NextREG 41 (29h) – PS/2 Keymap Address LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
PS/2 Keymap Address LSB		■	•	•	•	•	•	•	•	•	8-bit value (0-255)		0

NextREG 42 (2Ah) – PS/2 Keymap Data MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
PS/2 Keymap Data MSB		■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	PS/2 Keymap Data MSB*
Reserved		■	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	Reserved, must be 0

* Not currently used by hardware. Write 0

NextREG 43 (2Bh) – PS/2 Keymap Data LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
PS/2 Keymap Data LSB		■	•	•	•	•	•	•	•	•	8-bit value (0-255)

A write causes the data to be written and auto-increments the Keymap Address

NextREG 44 (2Ch) – DAC B Mirror (Left) / I²S Left Sample MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
I²S Left Sample MSB	■		•	•	•	•	•	•	•	•	8-bit value (0-255)
8-bit sample Left DAC B		■	•	•	•	•	•	•	•	•	8-bit value		*

* A soft reset sets a value of 128 (80h)
The I²S Left Sample LSB is latched and can be read from NextREG 45 (2Dh) later

NextREG 45 (2Dh) – DAC A+D Mirror (Mono) / I²S Sample LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
I²S Last Sample LSB	■		•	•	•	•	•	•	•	•	8-bit value (0-255)
8-bit sample DACs A + D		■	•	•	•	•	•	•	•	•	8-bit value		*

* A soft reset sets a value of 128 (80h)
Returns the LSB of last sample read from NextREG 44(2Ch) or NextREG 46(2Eh)

NextREG 46 (2Eh) – DAC C Mirror (Right) / I²S Right Sample MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
I²S Right Sample MSB	■		•	•	•	•	•	•	•	•	8-bit value (0-255)
8-bit sample Right DAC C		■	•	•	•	•	•	•	•	•	8-bit value		*

* A soft reset sets a value of 128 (80h)
The I²S Right Sample LSB is latched and can be read from NextREG 45 (2Dh) later

NextREG 47 (2Fh) – Layer 3 (Tilemap) Horizontal Scroll Control MSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Layer 3 X Scroll Offset MSB	■	■	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	colour: dark green	•	•	Tilemap X Scroll Offset MSB		0
Reserved		■	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: dark green 0	colour: pink	colour: pink	Reserved, must be 0

Meaningful range: 0 to 319 in 40 tiles mode, 0 to 639 in 80 tiles mode

NextREG 48 (30h) – Layer 3 (Tilemap) Horizontal Scroll Control LSB

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
X Offset LSB	■	■	•	•	•	•	•	•	•	•	8-bit value (0-255)		0

Meaningful range: 0 to 319 in 40 tiles mode, 0 to 639 in 80 tiles mode

NextREG 49 (31h) – Layer 3 (Tilemap) Vertical Scroll Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Y Offset	■	■	•	•	•	•	•	•	•	•	8-bit value (0-255)		0

NextREG 50 (32h) – Layer 1,0 (LoRes) Horizontal Scroll Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
X Offset	■	■	•	•	•	•	•	•	•	•	8-bit value of X Offset (0-255)		0

Layer 1,0 (LoRes) scrolls in half-pixels at the same resolution and smoothness as Layer 2

NextREG 51 (33h) – Layer 1,0 (LoRes) Vertical Scroll Control

Group Name	R	W	Data Bits								Description	H	D
Group Name	R	W	7	6	5	4	3	2	1	0	Description	H	D
Y Offset	■	■	•	•	•	•	•	•	•	•	8-bit value of Y Offset (0-191)		0

Layer 1,0 (LoRes) scrolls in half-pixels at the same resolution and smoothness as Layer 2

ZX Spectrum Next User Manual, 3rd Edition (ISBN 978-1-5272-5496-1), written and illustrated by Phoebus R. Dokos. Copyright © 2020-2024 Phoebus Dokos / SpecNext Ltd. Licensed under CC BY-NC-SA 4.0. This is a transcription and can contain errors; check any doubt against the printed page.