Save the current input-source specification. Store u in BLK (thus making block u the input source and setting the input buffer to encompass its contents), set >IN to zero, and interpret. When the parse area is exhausted, restore the prior input source specification. Other stack effects are due to the words LOADed.
An ambiguous condition exists if u is zero or is not a valid block number.
mcondron Getting the block contentsRequest for clarification2019-08-10 00:37:10
This word is surely intended to actually transfer the contents of the specified block from the mass storage device to a buffer, but this behavior is not described. From reading the description, it gives the impression that LOAD is meant to be run after a block has been transferred from mass storage. Is that the case? It's not what I get from reading historical documents on this -- it seems LOAD is meant to initiate a transfer of data.
Maybe what's intended is something like: "Save the current input source specification. Read block u from mass storage in the manner described in BLOCK. Make the block buffer where block u was saved be the input source. Set >IN to zero and interpret, then restore the prior input source specification."