What is the reason for the separate “print” call in the error handling in module 454?

Question

Here is the contents of module 454 (Scan for fil units; goto attach_fraction if found):

if scan_keyword ("fil") then
  begin
    cur_order ← fil;
    while scan_keyword ("l") do
      begin
        if cur_order = filll then
          begin
            print_err("Illegal unit of measure (");
            print("replaced by filll)");
            help1("I dddon´t go any higher than filll."); error;
          end
        else
          incr (cur_order);
      end;
    goto attach_fraction;
  end

When an erroneous additional l is detected, the program reports "Illegal unit of measure (replaced by filll)". Couldn't this be done simply by print_err("Illegal unit of measure (replaced by filll)")?

Answer 1

TeX's string handling is based around some limitations of 1970s computing (in particular, rather than letting some standard library handle allocations of strings, instead they're all kept in a statically allocated array which Knuth refers to as the string pool. See §50 for a note about this. The result is that the actual call in the generated Pascal code will not be, e.g., print("replaced by filll)" but actually print(447) (or whatever the actual string number is). The relevant code is in part 5 (§54–71).

As Barbara noted, Illegal Unit of measure ( makes three appearances in tex.web (you can check in the index in the output from weave). This allows a savings of a few bytes which in our current era when memory is generally measured in gigabytes, but it wasn't uncommon for the total addressable memory of a multi-user system, even into the 80s, to be measured in a small number of megabytes. There's a reason that the original TeX had 16-bit addressing of memory.