Bit0-7 Keyboard keys (dummy read from row 6 is done... to request/acknowledge something...?)
The exact mechanism that defines when the hardware does output the next 4bit fragment is unknown. The software does manipulate row 9 & row 6 signals, which does somehow request and/or acknowledge the fragments. Looking at the [[Media:CPC Mousepack 2.0 disassembly.txt|disassembled software]], the hardware seems to work like so (not confirmed):
switch from row0 to row9 --> invoke reading X counter (after 200 clks: MSB, after another 200 clks: LSB)
switch from row9 to row6 --> terminate X read