Difference between revisions of "Expansion Bus Control Register"

Latest revision as of 11:45, 3 November 2024

Number	$81
Readable	Yes
Writable	Yes
Short Description	Expansion bus controls

Bit	Description
7	1 if ROMCS is being asserted on the expansion bus (Read only, write 0)
6	1 to allow peripherals to override the ULA on some even port reads (rotronics wafadrive)
5	1 to disable expansion bus nmi debounce (opus discovery)
4	1 to propagate the max CPU clock at all times including when the expansion bus is off
1-0	max CPU speed when the expansion bus is on (currently fixed at 00 = 3.5MHz)

Set to 0 upon hard reset.

(new register since core 3.0.5)

(note: Next registers with number higher than $7F are inaccessible from Copper code)

@@ Line 5: / Line 5: @@
 |ShortDesc=Expansion bus controls
 }}
-Read (may change in the future):
 {|class="wikitable"
 ! Bit !! Description
 |-
-| 7 || ROMCS is being asserted on the expansion bus
+| 7 || 1 if ROMCS is being asserted on the expansion bus (Read only, write 0)
 |-
-| 6 || Expansion bus is currently enabled
+| 6 || 1 to allow peripherals to override the ULA on some even port reads (rotronics wafadrive)
 |-
-| 5 || Max clock is always propagated to the expansion bus
+| 5 || 1 to disable expansion bus nmi debounce (opus discovery)
 |-
-| 4 || 0 indicates the 48K rom is locked in place
+| 4 || 1 to propagate the max CPU clock at all times including when the expansion bus is off
 |-
-| 3-2 || Reserved
+| 1-0 || max CPU speed when the expansion bus is on (currently fixed at 00 = 3.5MHz)
-|-
-| 1-0 || Max cpu speed while the expansion bus is enabled
 |}
-Write (may change in the future):
+Set to 0 upon hard reset.
-{|class="wikitable"
-! Bit !! Description
+(new register since core 3.0.5)
-|-
-| 7 || Make change immediate otherwise changes are noted for next soft reset
-|-
-| 6 || Enable the expansion bus (hard reset = 0)
-|-
-| 5 || Always propagate the max clock to the expansion bus (hard reset = 0)
-|-
-| 4 || 0 to lock the 48k rom in place (hard reset = 0)
-|-
-| 3-2 || Reserved, must be 0
-|-
-| 1-0 || CPU speed when the expansion bus is enabled (currently fixed at 00 = 3.5 MHz) (hard reset = 00)
-|}
 (note: Next registers with number higher than $7F are inaccessible from Copper code)