Layer 2 Access Port - Revision history

Johnnyo: bold table titles

2026-03-18T21:49:33Z

bold table titles

← Older revision		Revision as of 22:49, 18 March 2026
Line 8:		Line 8:
	\|Subsystem=Layer 2		\|Subsystem=Layer 2
	}}		}}
	Read or write-with-bit-4-zero (soft reset = 0):		'''Read or write-with-bit-4-zero''' (soft reset = 0):

	{\| class="wikitable"		{\| class="wikitable"
Line 35:		Line 35:
	The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).		The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).

	Write-with-bit-4-set (since core 3.0.7) (soft reset = 0):		'''Write-with-bit-4-set''' (since core 3.0.7) (soft reset = 0):

	{\| class="wikitable"		{\| class="wikitable"

Ped7g at 11:02, 4 May 2020

2020-05-04T11:02:59Z

← Older revision		Revision as of 12:02, 4 May 2020
Line 1:		Line 1:
	{{Port		{{Port
	\|Number=$123B		\|Number=$123B
			\|NumberDec=4667
			\|PortMask=%0001 0010 0011 1011 ??
	\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.		\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.
	\|Readable=Yes		\|Readable=Yes
	\|Writable=Yes		\|Writable=Yes
	\|Subsystem=Layer 2		\|Subsystem=Layer 2
	~~\|PortMask=%0001 0010 0011 1011 ??~~
	}}		}}
	Read or write-with-bit-4-zero (soft reset = 0):		Read or write-with-bit-4-zero (soft reset = 0):

Ped7g: core 3.1.4 changes/refresh

2020-04-16T00:54:57Z

core 3.1.4 changes/refresh

← Older revision		Revision as of 01:54, 16 April 2020
Line 7:		Line 7:
	\|PortMask=%0001 0010 0011 1011 ??		\|PortMask=%0001 0010 0011 1011 ??
	}}		}}
	Read or write-with-bit-4-zero:		Read or write-with-bit-4-zero (soft reset = 0):

	{\| class="wikitable"		{\| class="wikitable"
Line 34:		Line 34:
	The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).		The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).

	Write-with-bit-4-set (since core 3.0.7):		Write-with-bit-4-set (since core 3.0.7) (soft reset = 0):

	{\| class="wikitable"		{\| class="wikitable"

Ped7g at 10:55, 31 December 2019

2019-12-31T10:55:29Z

← Older revision		Revision as of 11:55, 31 December 2019
Line 34:		Line 34:
	The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).		The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).

	Write-with-bit-4-set:		Write-with-bit-4-set (since core 3.0.7):

	{\| class="wikitable"		{\| class="wikitable"

Ped7g: core 3.0.7 changes/refresh

2019-12-31T10:54:15Z

core 3.0.7 changes/refresh

← Older revision		Revision as of 11:54, 31 December 2019
Line 7:		Line 7:
	\|PortMask=%0001 0010 0011 1011 ??		\|PortMask=%0001 0010 0011 1011 ??
	}}		}}
			Read or write-with-bit-4-zero:

	{\| class="wikitable"		{\| class="wikitable"
	! Bit !! Description		! Bit !! Description
	\|-		\|-
	\| 7-6 \|\| Video RAM bank select (write/read paging)		\| 7-6 \|\| Video RAM bank select (write/read paging)
			00 = first 16K of layer 2 in the bottom 16K
			01 = second 16K of layer 2 in the bottom 16K
			10 = third 16K of layer 2 in the bottom 16K
			11 = first 48K of layer 2 in the bottom 48K (since core 3.0)
	\|-		\|-
	\| 5-4 \|\| Reserved, write 0		\| 5 \|\| Reserved, write 0
	\|-		\|-
	\| 3 \|\| ~~Use Shadow Layer 2 for paging - {{NextRegNo\|$13}}~~		\| 4 \|\| 0
	\|-		\|-
	\| 2 \|\| Enable ~~Layer 2 read-only paging~~		\| 3 \|\| 0 = map {{NextRegNo\|$12}}, 1 = map {{NextRegNo\|$13}}
			\|-
			\| 2 \|\| Enable mapping for memory reads
	\|-		\|-
	\| 1 \|\| Layer 2 visible - {{NextRegNo\|$12}}		\| 1 \|\| Layer 2 visible - {{NextRegNo\|$12}}
	Since core 3.0 this bit has mirror in {{NextRegNo\|$69}}		Since core 3.0 this bit has mirror in {{NextRegNo\|$69}}
	\|-		\|-
	\| 0 \|\| Enable Layer 2 write-~~only paging~~		\| 0 \|\| Enable mapping for memory writes
			\|}

			The Layer 2 data being displayed are always driven by {{NextRegNo\|$12}}, the "shadow" register $13 does affect only the memory mapping feature (when bit 3 is set), never the display data (to "flip" the data being displayed, write new bank value into register $12).

			Write-with-bit-4-set:

			{\| class="wikitable"
			! Bit !! Description
			\|-
			\| 7-5 \|\| Reserved, write 0
			\|-
			\| 4 \|\| 1
			\|-
			\| 3 \|\| Reserved, write 0
			\|-
			\| 2-0 \|\| 16ki bank relative offset (+0 .. +7) applied to Layer 2 memory mapping
	\|}		\|}

			The bit 4 = 1 functionality was added in core 3.0.7 together with new Layer 2 display modes, most likely usage is to set up mapping like %00'00'??'1'? or %11'00'??'1'? to map first bank into bottom 16ki or 48ki of memory, then switching the mapped bank by writing relative offset value 0..7 to the port with bit 4 set, like %0001'0011 to modify mapping to start with fourth (+3) bank of Layer 2. The original mapping mode did cover only first three banks (which covers whole 256x192 Layer 2 mode), but the new Layer 2 modes 320x256x8bpp and 640x256x4bpp occupy full five 16kiB banks, this new relative offsetting can be used to reach any of the five banks even when "first bank into bottom 16ki" type of mapping is selected.

Ped7g: core 3.0 changes/refresh

2019-11-27T19:17:51Z

core 3.0 changes/refresh

@@ Line 2: / Line 2: @@
 |Number=$123B
 |ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.
-|Readable=No
+|Readable=Yes
-|Writable=No
+|Writable=Yes
 |Subsystem=Layer 2
 |PortMask=%0001 0010 0011 1011 ??
 }}

Xalior at 10:14, 20 April 2019

2019-04-20T10:14:52Z

← Older revision		Revision as of 11:14, 20 April 2019
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	{{~~#set:~~Port		{{Port
	\|Number=$123B		\|Number=$123B
	\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.		\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.

Xalior at 10:05, 20 April 2019

2019-04-20T10:05:06Z

← Older revision		Revision as of 11:05, 20 April 2019
Line 1:		Line 1:
	{{Port		{{#set:Port
	\|Number=$123B		\|Number=$123B
	\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.		\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.

Xalior at 10:04, 20 April 2019

2019-04-20T10:04:16Z

← Older revision		Revision as of 11:04, 20 April 2019
Line 1:		Line 1:
	{{~~#set:~~		{{Port
	Port
	\|Number=$123B		\|Number=$123B
	\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.		\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.

Xalior at 10:03, 20 April 2019

2019-04-20T10:03:31Z

← Older revision		Revision as of 11:03, 20 April 2019
Line 1:		Line 1:
	{{Port		{{#set:
			Port
	\|Number=$123B		\|Number=$123B
	\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.		\|ShortDesc=Enables [[Layer 2]] and controls paging of layer 2 screen into lower memory.