366 THE PDP-11 FAMILY the PDP-11/20 instructions provided only
single-bit shifts.) The first disk-based operating system, DOS,
was designed for a minimum standard system that included 8
Kwords (16 Kbytes) of memory. After allowing typically 2 Kwords
for the resident parts of the monitor, only 5 K to 6 K remained
for other use. Consequently, size constraints played a major
role in the FORTRAN system design and implementation. There were
not many competitors at the time, but at least one, the IBM
1130, offered a disk-based operating system and FORTRAN system.
To meet this competition, an important goal was to deliver the
PDP-l1 FORTRAN system to the market as quickly as possible, even
at the cost of performance, if necessary. Neither Compiler nor
Interpreter, but Threaded Code The fundamental design strategy
to be determined was the structure of the executing code, the
"run-time environment" [DEC, 1974b; DEC, 1974c]. We were leery
of a compiler that generated direct machine code primarily
because of the size of