Difference between revisions of "Keymap High Address Register"

Latest revision as of 11:35, 3 November 2024

Number	$28
Readable	Yes
Writable	Yes
Short Description	PS/2 Keymap address MSB, read (pending) first byte of palette colour

Write:

 bit 7 = 0 to select ps2 keymap, 1 to select key joystick
 bits 6:1 = Reserved, must be 0
 bit 0 = MSB keymap address

Read:

 bits 7:0 = Stored first-byte palette value from Enhanced ULA Palette Extension ($44)

This value does update right after writing first 8 bits of colour to NextReg $44, while the real palette entry is modified only after the second write (adding ninth bit and L2 priority bit) is done. So half-written value can't be read back through Palette Value Register ($41), but can be read with this register (after writing second byte to $44, this register still reads the first byte value, but the change is also committed to the palette itself and visible by reading $41 + $44 registers).

@@ Line 1: / Line 1: @@
 {{NextRegister
 |Number=$28
-|Readable=No
+|Readable=Yes
 |Writable=Yes
-|ShortDesc=High address of Keymap (MSB in bit 0)
+|ShortDesc=PS/2 Keymap address MSB, read (pending) first byte of palette colour
 }}
-Bits 7-1 are reserved, must be 0. Bit 0 is MSB of keymap address.
+Write:
+  bit 7 = 0 to select ps2 keymap, 1 to select key joystick
+  bits 6:1 = Reserved, must be 0
+  bit 0 = MSB keymap address
+Read:
+  bits 7:0 = Stored first-byte palette value from {{NextRegNo|$44}}
+This value does update right after writing first 8 bits of colour to NextReg $44, while the real palette entry is modified only after the second write (adding ninth bit and L2 priority bit) is done. So half-written value can't be read back through {{NextRegNo|$41}}, but can be read with this register (after writing second byte to $44, this register still reads the first byte value, but the change is also committed to the palette itself and visible by reading $41 + $44 registers).