I wonder if there is a mistake on Thierry's page?
"Placing the TMS9901 in I/O mode again will resume updating of the Read register. However, if any bit between 1 and 14 is written to while in timer mode, the decrementer will be reinitialized with the current value of the Clock register. This is nice because it means that it is not necessary to reload all 14 bits in the Clock register: since they haven't changed, writing one of them (such as the least significant one) is enough to reload the whole data word."
Should the text I have marked in bold read "I/O mode" instead?
No. You can read from the Read register and write to the Clock register only in timer mode (called Clock mode in TMS9901 Data Manual). The reason that writing to one bit “is enough to reload the whole data word” is that the clock is reloaded/restarted for every bit that is written as it is written, i.e., if you write all 14 bits, the clock is re-initialized 14 times. Below is an excerpt from the TMS9901 Data Manual (page 8, ¶2):
TMS9901P8p2.png 202.4KB 2 downloads