 The FORTRAN system is very compact, having been designed and implemented by Digitek for SDS to compile and run in 4,096 word SDS 900 series machines.To do anything useful in such small memory space, the compiler relies on an SDS architectural feature known as Programmed OPeratorS, or POPS.This feature consists of a single bit in the instruction word that causes the machine to "mark place and branch" to the memory address of the instruction code value plus 100 (octal).