A description of the Incremental System Programming Language (ISPL). This highly interactive, one-language, time-sharing system is designed around an incremental compiler that will support many private subsystems; it uses a flexible, high-speed, microcoded machine with read/write microstore capability. Not yet implemented, the ISPL system nevertheless offers a powerful approach to system building through its concurrent design of language, operating system, and machine. This approach (1) simplified the incremental compilation by localizing the effects of modifications, and simplified the addressing structure by changing virtual addresses into real ones in two microcode cycles; (2) centralized all communication through "ports" that can be manipulated either externally or internally--thereby enabling the construction of hierarchical subsystems; (3) separated the maintenance from the scheduling of resources so that subsystems could schedule the resources while system integrity was ensured; and (4) simplified the system design through a greatly expanded machine structure in which many software functions were included as microcode. 18 pp. Ref.