Keymap High Address Register

From SpecNext official Wiki
Jump to: navigation, search
Number $28
Readable Yes
Writable Yes
Short Description PS/2 Keymap address MSB, read (pending) first byte of palette colour


 bits 7:1 = Reserved, must be 0
 bit 0 = MSB keymap address


 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).