After looking at the code, it looks like Talarm(), opcode 0x120, may only have a granularity of seconds according to the comments in the kernel, which isn't what the Compendium says. Could someone confirm? It looks like there is a new call, Tmalarm(), opcode 0x13d, for millisecond granularity.
The Compendium is actually full of such mistakes, I don't see how anyone can regard it as a trustworthy documentation, if almost everything the Compendium says should be verified against Atari documentation and MiNT source code.
KMK