TAOCP 1.3.1 Exercise 15
In MIX, each alphanumeric character is represented by one byte, since a byte holds at least $64$ distinct values and is used for character coding in input-output operations.
Section 1.3.1: Description of MIX
Exercise 15. [10] How many alphanumeric characters are there in a typewriter or paper-tape block? in a card-reader or card-punch block? in a line-printer block?
Verified: yes
Solve time: 36s
In MIX, each alphanumeric character is represented by one byte, since a byte holds at least $64$ distinct values and is used for character coding in input-output operations. Thus the number of characters in a block equals the number of bytes in that block.
A typewriter or paper-tape block contains $100$ bytes, hence it contains $100$ alphanumeric characters. A card-reader or card-punch block corresponds to a single card with $80$ columns, hence it contains $80$ alphanumeric characters. A line-printer block contains one printed line of $120$ positions, hence it contains $120$ alphanumeric characters.
Thus the respective answers are
\boxed{100,\ 80,\ 120}. $$∎