UPD7220 Video Card (Revision 2) - Revision history

Nuclear: Formatting fixes

2024-06-14T04:49:19Z

Formatting fixes

Nuclear: Removed a weird comment from the example code

2024-06-14T04:41:44Z

Removed a weird comment from the example code

Nuclear: Checkpoint: finished the initialization section at last!

2024-06-14T04:37:58Z

Checkpoint: finished the initialization section at last!

Nuclear: Software protocol checkpoint

2024-06-14T04:17:08Z

Software protocol checkpoint

Nuclear: Finish video modes section

2024-06-14T01:20:27Z

Finish video modes section

Nuclear: Video modes tutorial; checkpoint/saving work

2024-06-14T01:08:53Z

Video modes tutorial; checkpoint/saving work

Nuclear: Fix table formatting

2024-06-14T00:27:23Z

Fix table formatting

Nuclear: Checkpoint for programming guide and video mode information

2024-06-14T00:22:35Z

Checkpoint for programming guide and video mode information

Nuclear: Create UPD7220 Video Card (Revision 2) page

2024-06-13T22:49:48Z

Create UPD7220 Video Card (Revision 2) page

New page

The uPD7220 Video Card revision 2 is a video card with 128KB of VRAM using the uPD7220 GDC or its variants. It operates at a maximum clock speed of 32Mhz. The card is based on the original uPD7220 card I (Dylan Brophy) made, which had half the VRAM and generally was far simpler. This card solves several issues with the older card, at the cost of greater size and complexity.

== Detailed Specifications ==

* 16-color VGA output
* 128KB of RAM
** 4 RAM chips, 32KB each, 25ns access and write time
** 16 bit bus (8 bits per chip, two chips per byte)
** 4 color bits per pixel, 4 pixels per word in RAM
*** Maximum 262144 pixels (hardware double buffering disabled)
**** Suggested resolutions: 544x480, 640x408
*** Maximum 131072 pixels (hardware double buffering enabled)
**** Suggested resolutions: 448x288, 400x300
* uPD7220 at up to 24Mhz, Z7220 at up to 32Mhz
* uPD7220 wide mode supported
** Allows higher pixel clock frequencies, for higher resolutions and lower visible display times
** Accesses two VRAM words per uPD7220 clock cycle
* Direct access to VRAM is provided to the host CPU

@@ Line 249: / Line 249: @@
 You can read the [http://www.vintagecomputer.net/fjkraan/comp/qx10/doc/nec7220.pdf programming manual] to get a detailed understanding of how to program the uPD7220, but I will also provide an abridged
 version here.  Once the card's configuration register is set, the uPD7220 must be initialized.  The uPD7220 requires a sequence of commands to be sent to set it up:
-. A reset command (write 0x00 to address 0x01)
+# A reset command (write 0x00 to address 0x01)
-** This must be followed by parameter bytes, described below
+#* This must be followed by parameter bytes, described below
-. Setting the video mode to master (write 0x6f to address 0x01)
+# Setting the video mode to master (write 0x6f to address 0x01)
-. Initializing the PRAM
+# Configuring the pitch
-** PRAM is some internal memory in the uPD7220, which configures how it accesses memory, and probably some other things
+#* PRAM is some internal memory in the uPD7220, which configures how it accesses memory, and probably some other things
-. CCHAR command to configure the character characteristics
+# CCHAR command to configure the character characteristics
-. Starting the display (write 0x6B to 0x01)
+# Disabling zoom (write 0x46 to 0x01, then 0x00 to 0x00)
-. Disable blanking (write 0x0D to 0x01)
+# Starting the display (write 0x6B to 0x01)
 My driver also seems to issue a FIGS command at the end of startup, but this may not be necessary.
@@ Line 373: / Line 374: @@
 My driver executes a FIGS command last, although this probably is not necessary.  Just in case, here is the command sequence:
-. 0x02 to address 0x00
+# 0x4C to address 0x01
-. 0x00 to address 0x00
+# 0x02 to address 0x00
-. 0x00 to address 0x00
+# 0x00 to address 0x00
 Here is my code which executes this exact sequence:

@@ Line 405: / Line 405: @@
   write(0, 0x00);
   write(0, (VERT_PIXELS << 4) & 255);
-  write(0, (VERT_PIXELS >> 4) | (WIDE_MODE ? 8 : 0));//*/
+  write(0, (VERT_PIXELS >> 4) | (WIDE_MODE ? 8 : 0));
   // Set character and cursor characteristics

← Older revision		Revision as of 04:37, 14 June 2024
Line 333:		Line 333:

	The fourth command is a PRAM command, which writes data to the uPD7220 parameter RAM. Here is a recommended usage of the command (although other usages are possible, again read the manual for more info):		The fourth command is a PRAM command, which writes data to the uPD7220 parameter RAM. Here is a recommended usage of the command (although other usages are possible, again read the manual for more info):
		+
		+	{\| class="wikitable" style="width:50%"
		+	\|+ The PRAM Command
		+	\|-
		+	! Address !! Value / Description
		+	\|-
		+	\| 1 \|\| 0x70 (starts PRAM write at PRAM byte 0)
		+	\|-
		+	\| 0 \|\| Display partition 1 starting address (lower 8 bits) - it is recommended to leave this as zero.
		+	\|-
		+	\| 0 \|\| Display partition 1 starting address (bits 8-16) - it is recommended to leave this as zero.
		+	\|-
		+	\| 0 \|\| Lower 2 bits are the display partition 1 starting address (upper 2 bits). Upper 4 bits are the lower 4 bits of the display partition length, in horizontal lines (NOT words). Bits 2 and 3 should be left as zeros.
		+	\|-
		+	\| 0 \|\| Lower 6 bits are the upper 6 bits of the image partition length, in horizontal lines (NOT words). Bit 6 (0x40) indicates it is a graphics area, otherwise it is a character area if in mixed mode. Bit 7 (0x80) enables wide mode - ensure to set this bit if you are using wide mode.
		+	\|}
		+
		+	The parameter RAM has more variables, but only these really need to be specified. There is another display partition that one can use, and configuration options for drawing and characters, later in the PRAM.
		+
		+	If your display mode uses wide mode, then be sure to set the MSB of the fourth parameter byte of this command, otherwise the memory access will be incorrect for wide mode.
		+
		+	Next, the CCHAR command is used to define the cursor and character characteristics:
		+
		+	{\| class="wikitable" style="width:50%"
		+	\|+ The CCHAR Command
		+	\|-
		+	! Address !! Value / Description
		+	\|-
		+	\| 1 \|\| 0x4B
		+	\|-
		+	\| 0 \|\| Setting the MSB will enable the cursor in character mode. The lower 5 bits are the number of lines per character row minus one. My driver leaves this byte as zero.
		+	\|-
		+	\| 0 \|\| Lower 5 bits define the cursor top line number in the row. Bit 5 makes the cursor steady if set, or blinking if reset. Upper two bits are the lower two bits of the blink rate. My driver leaves this byte as 0xC0.
		+	\|}
		+
		+	This is sufficient for non-interlaced modes, but interlaced modes require this command to get a third byte.
		+
		+	Zoom must be disabled with a 0x46 write to address 0x01, followed by writing 0x00 to address 0x00. At this point we can finally enable the display and disable blanking by writing commands 0x6B, then 0x0D, to address 0x01.
		+
		+	My driver executes a FIGS command last, although this probably is not necessary. Just in case, here is the command sequence:
		+	1. 0x4C to address 0x01
		+	2. 0x02 to address 0x00
		+	3. 0x00 to address 0x00
		+	4. 0x00 to address 0x00
		+
		+	Here is my code which executes this exact sequence:
		+
		+	<pre>
		+	void setup_upd7220() {
		+	// Reset command
		+	write(1, 0);
		+	write(0, 0x10); // operation mode: draw only during blanking
		+	write(0, HORIZ_WORDS - 2);
		+	write(0, (VERT_SYNC << 5) \| (HORIZ_SYNC - 1));
		+	write(0, ((HORIZ_FRONT_PORCH - 1) << 2) \| (VERT_SYNC >> 3));
		+	write(0, (HORIZ_BACK_PORCH - 1) & 63);
		+	write(0, VERT_FRONT_PORCH & 63);
		+	write(0, VERT_PIXELS & 255);
		+	write(0, (VERT_BACK_PORCH << 2) \| (VERT_PIXELS >> 8));
		+
		+	// set master video mode
		+	write(1, 0x6f);
		+
		+	// set pitch
		+	write(1, 0x47);
		+	write(0, HORIZ_WORDS);
		+
		+	// set some PRAM values
		+	write(1, 0x70);
		+	write(0, 0x00);
		+	write(0, 0x00);
		+	write(0, (VERT_PIXELS << 4) & 255);
		+	write(0, (VERT_PIXELS >> 4) \| (WIDE_MODE ? 8 : 0));//*/
		+
		+	// Set character and cursor characteristics
		+	write(1, 0x4B);
		+	write(0, 0x00);
		+	write(0, 0xC0);
		+
		+	// set zoom
		+	write(1, 0x46);
		+	write(0, 0x00);
		+
		+	// Enable the display
		+	write(1, 0x6b);
		+
		+	// Disable blanking
		+	write(1, 0x0D);
		+
		+	// This is probably not necessary
		+	write(1, 0x4c);
		+	write(0, 0x02);
		+	write(0, 0x00);
		+	write(0, 0x00);
		+	}
		+	</pre>

← Older revision		Revision as of 04:17, 14 June 2024
Line 244:		Line 244:
	\|}		\|}

−	It is recommended to keep track of what buffer is currently displaying, and perhaps this entire configuration register, so that the selected buffer can be switched. This register is important for controlling how the CPU accesses VRAM, how smooth the display is, and the way the image data is displayed.	+	It is recommended to keep track of what buffer is currently displaying, and perhaps this entire configuration register, so that the selected buffer can be switched. This register is important for controlling how the CPU accesses VRAM, how smooth the display is, and the way the image data is displayed. Generally, automatic double buffering is recommended if the resolution demands less than half of the available VRAM, as
		+	double buffering makes the display smoother by removing visual artifacts due to drawing, but doubles VRAM demand.
		+
		+	You can read the [http://www.vintagecomputer.net/fjkraan/comp/qx10/doc/nec7220.pdf programming manual] to get a detailed understanding of how to program the uPD7220, but I will also provide an abridged
		+	version here. Once the card's configuration register is set, the uPD7220 must be initialized. The uPD7220 requires a sequence of commands to be sent to set it up:
		+	1. A reset command (write 0x00 to address 0x01)
		+	** This must be followed by parameter bytes, described below
		+	2. Setting the video mode to master (write 0x6f to address 0x01)
		+	3. Configuring the pitch
		+	4. Initializing the PRAM
		+	** PRAM is some internal memory in the uPD7220, which configures how it accesses memory, and probably some other things
		+	5. CCHAR command to configure the character characteristics
		+	6. Disabling zoom (write 0x46 to 0x01, then 0x00 to 0x00)
		+	7. Starting the display (write 0x6B to 0x01)
		+	8. Disable blanking (write 0x0D to 0x01)
		+
		+	My driver also seems to issue a FIGS command at the end of startup, but this may not be necessary.
		+
		+	Each command starts with a write to address 0x01, and for each parameter a write to address 0x00 follows. Each command I describe will proceed this way, with the first byte being the command byte written to
		+	address 0x01, and all following bytes being parameters written to address 0x00. At the end I will have some example code in C which provides as both an example and an additional explanation.
		+
		+	The first command is the reset command; here is the format:
		+
		+	{\| class="wikitable" style="width:50%"
		+	\|+ The Reset Command
		+	\|-
		+	! Address !! Value / Description
		+	\|-
		+	\| 1 \|\| 0x00
		+	\|-
		+	\| 0 \|\| Operation mode. Discussed below.
		+	\|-
		+	\| 0 \|\| Horizontal display words minus two. Must be an even number. This is equal to ((horizontal_pixels / 4) - 2). Wide mode does not affect this.
		+	\|-
		+	\| 0 \|\| Lower 5 bits are the HSync width minus 1, in words. Upper 3 bits are the lower 3 bits of the VSync width.
		+	\|-
		+	\| 0 \|\| Lower 2 bits are the upper 2 bits of the VSync width. Upper 6 bits are the horizontal front porch width minus 1.
		+	\|-
		+	\| 0 \|\| Horizontal back porch width minus one. Upper two bits must be zero.
		+	\|-
		+	\| 0 \|\| Vertical front porch width minus one. Upper two bits must be zero.
		+	\|-
		+	\| 0 \|\| Number of display lines per frame, lower 8 bits.
		+	\|-
		+	\| 0 \|\| Lower 2 bits are the upper 2 bits of the number of display lines per frame. Upper 6 bits are the vertical back porch length.
		+	\|}
		+
		+	Note that the vertical field sizes do not have numbers subtracted from them, but the horizontal field sizes have one or two subtracted from them. If this description is confusing, the uPD7220 manual provided above does contain a nice illustration you may consider looking at.
		+
		+	The operation mode is a bitmask, which controls interlacing, character or graphic mode, as well as DRAM refresh and if drawing is allowed during an active display cycle. Here are some of the options that
		+	one can OR together to control the operation mode:
		+
		+	{\| class="wikitable" style="width:50%"
		+	\|+ Operation control bits
		+	\|-
		+	! Bitmask !! Selected Option
		+	\|-
		+	\| 0x00 \|\| Leave bits 5 and 1 zero to have mixed character and graphics mode.
		+	\|-
		+	\| 0x02 \|\| Enables just graphics mode
		+	\|-
		+	\| 0x20 \|\| Enables just character mode
		+	\|-
		+	\| 0x00 \|\| Leave bits 3 and 0 zero to disable interlacing
		+	\|-
		+	\| 0x08 \|\| Enables interlacing
		+	\|-
		+	\| 0x09 \|\| Enables "Interlaced repeat field for character displays". Read the manual for more information.
		+	\|-
		+	\| 0x04 \|\| Enables DRAM refresh cycles - not used on this uPD7220 card, as static RAM is used.
		+	\|-
		+	\| 0x10 \|\| Disables drawing during active display time. Enabling this is recommended.
		+	\|}
		+
		+	Generally 0x12 is a good setting for this card, unless you intend on using character mode. 0x10 will also work as a good mode.
		+
		+	The second command one must send is 0x6f, again written to address 0x01, and it does not take any parameters. The third command is the PITCH command:
		+
		+	{\| class="wikitable" style="width:50%"
		+	\|+ The Pitch Command
		+	\|-
		+	! Address !! Value / Description
		+	\|-
		+	\| 1 \|\| 0x47
		+	\|-
		+	\| 0 \|\| Number of memory words per line. Usually the same as the number of display words per line. Generally equal to (horizontal_pixels / 4). The only reason it would differ, is if the
		+	\|}
		+
		+	The fourth command is a PRAM command, which writes data to the uPD7220 parameter RAM. Here is a recommended usage of the command (although other usages are possible, again read the manual for more info):

@@ Line 36: / Line 36: @@
 ! Region !! Pixels !! Microseconds
 |-
-| Visible area || 640 || 25.422045680238
+| Visible area || 640 || 25.4220
 |-
-| Front porch || 16 || 0.63555114200596
+| Front porch || 16 || 0.6356
 |-
-| Sync pulse || 96 || 3.8133068520357
+| Sync pulse || 96 || 3.8133
 |-
-| Back porch || 48 || 1.9066534260179
+| Back porch || 48 || 1.9067
 |-
-| Whole line || 800 || 31.777557100298
+| Whole line || 800 || 31.778
 |}
@@ Line 50: / Line 50: @@
 |+ Vertical Timing
 |-
-! Region !! Lines !! Milliseconds
+! Region !! Lines
 |-
-Visible area || 480 || 15.253227408143
+| Visible area || 480
 |-
-Front porch || 10 || 0.31777557100298
+| Front porch || 10
 |-
-Sync pulse || 2 || 0.063555114200596
+| Sync pulse || 2
 |-
-Back porch || 33 || 1.0486593843098
+| Back porch || 33
 |-
-Whole frame || 525 || 16.683217477656
+| Whole frame || 525
 |}
@@ Line 81: / Line 81: @@
 | Visible area || 16.67
 |-
-| Front porch || 0.63555114200596
+| Front porch || 0.6356
 |-
-| Sync pulse || 3.8133068520357
+| Sync pulse || 3.8133
 |-
-| Back porch || 10.662032439589677
+| Back porch || 10.6620
 |-
-| Whole line || 31.777557100298
+| Whole line || 31.7776
 |}
@@ Line 98: / Line 98: @@
 ! Region !! Lines
 |-
-Visible area || 400
+| Visible area || 400
 |-
-Front porch || 60
+| Front porch || 60
 |-
-Sync pulse || 2
+| Sync pulse || 2
 |-
-Back porch || 63
+| Back porch || 63
 |-
-Whole frame || 525
+| Whole frame || 525
 |}
@@ Line 113: / Line 113: @@
 At this point we have the description of the mode we want, now we can convert it into the numbers used by the card.  The vertical timings are already in the correct format, but the horizontal timing still must
 '''Card Specific Details: Wide Mode'''

@@ Line 24: / Line 24: @@
 The uPD7220 was invented before the VGA standard, and due to this, was not designed to output a VGA signal.  Since VGA was designed to drive CRT monitors, as was the uPD7220, it turns out that
 '''Card Specific Details: Wide Mode'''
@@ Line 32: / Line 121: @@
 Due to wide mode support, the card's clock is 4x the uPD7220's clock.  When doing calculations for video, this must be kept in mind.  If wide mode is disabled, then the pixel clock is only half the card's clock.  If the video timing math does not account for this, then the video signal timing will be incorrect.
 == Communication with the Host Processor ==

@@ Line 44: / Line 44: @@
 Here is a pinout of the card:
-{| class="wikitable" style="margin:auto"
+{| class="wikitable"
 |+ Card Pinout
 |-
@@ Line 93: / Line 93: @@
 === Software Protocol ===
-{| class="wikitable" style="margin:auto"
+{| class="wikitable"
 |+ Card Registers
 |-
@@ Line 117: / Line 117: @@
 For this card, a driver should first configure register 2 for the selected video mode and card interface configuration.  This register has various bits to tell the card what to do:
-{| class="wikitable" style="margin:auto"
+{| class="wikitable" style="width:50%"
 |+ Configuration Register Bits
 |-

← Older revision		Revision as of 00:22, 14 June 2024
Line 18:		Line 18:
	** Accesses two VRAM words per uPD7220 clock cycle		** Accesses two VRAM words per uPD7220 clock cycle
	* Direct access to VRAM is provided to the host CPU		* Direct access to VRAM is provided to the host CPU
		+
		+	== Video Modes ==
		+
		+	It is assumed that the reader is familiar with VGA display timings. If not, then it is recommended to study it, or to use the video timing calculator and to leave the card in wide mode.
		+
		+	The uPD7220 was invented before the VGA standard, and due to this, was not designed to output a VGA signal. Since VGA was designed to drive CRT monitors, as was the uPD7220, it turns out that
		+	they are fairly compatible.
		+
		+	'''Card Specific Details: Wide Mode'''
		+
		+	This card can reach quite high clock speeds for the uPD7220, and to maintain this high speed for such slow chips, wide mode is used. This allows twice as many pixels to be output per uPD7220 clock cycle. This can be used either to increase the maximum resolution, or to reduce the display time. Reducing the display time gives the uPD7220 and host processor more time to draw to the screen. On the other hand, due to
		+	the high speeds used in wide mode, there is also typically more data that must be written to the display per frame, thus putting more pressure on the host processor. The balance in display characteristics depends on the application.
		+
		+	Due to wide mode support, the card's clock is 4x the uPD7220's clock. When doing calculations for video, this must be kept in mind. If wide mode is disabled, then the pixel clock is only half the card's clock. If the video timing math does not account for this, then the video signal timing will be incorrect.
		+
		+
		+
		+
		+	== Communication with the Host Processor ==
		+
		+	The hardware connection uses a fairly simple 8-bit parallel bus for data exchange. Once connected, the software can read from and write to several registers and addresses on the card to control it.
		+
		+	=== Hardware Protocol ===
		+
		+	Here is a pinout of the card:
		+
		+	{\| class="wikitable" style="margin:auto"
		+	\|+ Card Pinout
		+	\|-
		+	! Pin !! Name !! Description
		+	\|-
		+	\| 1 \|\| A0 \|\| Address pin 0 (LSB)
		+	\|-
		+	\| 2 \|\| A1 \|\| Address pin 1
		+	\|-
		+	\| 3 \|\| A2 \|\| Address pin 2 (MSB)
		+	\|-
		+	\| 4 \|\| NC \|\| No connection: this is the key pin and should be blocked
		+	\|-
		+	\| 5 \|\| GND \|\| Ground connection
		+	\|-
		+	\| 6 \|\| D0 \|\| Data pin 0 (LSB)
		+	\|-
		+	\| 7 \|\| D1 \|\| Data pin 1
		+	\|-
		+	\| 8 \|\| D2 \|\| Data pin 2
		+	\|-
		+	\| 9 \|\| D3 \|\| Data pin 3
		+	\|-
		+	\| 10 \|\| D4 \|\| Data pin 4
		+	\|-
		+	\| 11 \|\| D5 \|\| Data pin 5
		+	\|-
		+	\| 12 \|\| D6 \|\| Data pin 6
		+	\|-
		+	\| 13 \|\| D7 \|\| Data pin 7 (MSB)
		+	\|-
		+	\| 14 \|\| SEL \|\| Select pin (active low input) - selects the card for communication
		+	\|-
		+	\| 15 \|\| 5v \|\| +5V rail to power the card
		+	\|-
		+	\| 16 \|\| RD \|\| Read enable pin (active low input)
		+	\|-
		+	\| 17 \|\| WR \|\| Write enable pin (active low input)
		+	\|-
		+	\| 18 \|\| WAIT \|\| Wait indication (active low output)
		+	\|}
		+
		+
		+	When trying to access VRAM, if the VRAM is being used by the card, then WAIT will be pulled low to tell the CPU not to end the current operation until after the WAIT pin goes high again. After WAIT goes high, wait at least 31.25ns for the read/write operation to complete before releasing any control lines or changing any data/address lines. Never read data while WAIT is low, and wait 31.25ns after WAIT goes high before reading the data to the CPU. WAIT only becomes low when trying to read from or write to VRAM; otherwise WAIT is always high.
		+
		+	Each access cycle should last at least 31.25ns, and SEL should be de-asserted between cycles. Any conventional processor like Z80, 8086, 6502, etc, should have no issue with WAIT or access cycles below an operating frequency of 32Mhz, regardless of the speed of the card. There may be exceptions to this, but there are no known exceptions.
		+
		+	=== Software Protocol ===
		+
		+	{\| class="wikitable" style="margin:auto"
		+	\|+ Card Registers
		+	\|-
		+	! Register !! Purpose
		+	\|-
		+	\| 0 \|\| uPD7220 command register (uPD7220 A0 = 0)
		+	\|-
		+	\| 1 \|\| uPD7220 parameter register (uPD7220 A0 = 1)
		+	\|-
		+	\| 2 \|\| Card configuration register (write only)
		+	\|-
		+	\| 3 \|\| Direct VRAM access (accessing this may cause WAIT to go low)
		+	\|-
		+	\| 4 \|\| VRAM Address bits 0-7 (write only)
		+	\|-
		+	\| 5 \|\| Reserved - for now same as register 4
		+	\|-
		+	\| 6 \|\| VRAM Address bits 8-15 (write only)
		+	\|-
		+	\| 7 \|\| Reserved - for now same as register 6
		+	\|}
		+
		+	For this card, a driver should first configure register 2 for the selected video mode and card interface configuration. This register has various bits to tell the card what to do:
		+
		+	{\| class="wikitable" style="margin:auto"
		+	\|+ Configuration Register Bits
		+	\|-
		+	! Bit !! Purpose
		+	\|-
		+	\| 0 \|\| Enables automatic counting of the VRAM address registers if set. If reset, then the VRAM address registers do not change when VRAM is written. More on this later.
		+	\|-
		+	\| 1 \|\| Setting this bit enables the card's wide mode circutry. When reset, one pixel will be output every other clock cycle. This should match the setting on the uPD7220 IC.
		+	\|-
		+	\| 2 \|\| Enables automatic double buffering. When set, draws to VRAM from the host CPU or the uPD7220 will happen on the opposite buffer as the one displaying. Setting this halfs the available VRAM for displaying.
		+	\|-
		+	\| 3 \|\| Reserved
		+	\|-
		+	\| 4 \|\| Reserved
		+	\|-
		+	\| 5 \|\| Reserved
		+	\|-
		+	\| 6 \|\| Reserved
		+	\|-
		+	\| 7 \|\| Controls which buffer is displaying. Changing this bit will swap the currently displaying buffer. Only works in automatic double buffer mode.
		+	\|}
		+
		+	It is recommended to keep track of what buffer is currently displaying, and perhaps this entire configuration register, so that the selected buffer can be switched. This register is important for controlling how the CPU accesses VRAM, how smooth the display is, and the way the image data is displayed.