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THE EVOLUTION OF THE DECsystem-10 493
Table 2. Initial Goals, Constraints, and Basic Design Decisions
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User/Language/Operating System
- Cheap cost/user via timesharing without inconvenience of batch processing
- Timeshared use via terminals with protection between users
- Independent user machines to execute from any location in physical memory
- Unrestricted use of devices, e.g.. full-duplex use of terminals
- Support for wide range of compiled and interpreted languages
- No special batch mode, batch must appear like terminal via a command file
- Device-independent I/O so that programs would run on different configurations and I/O could be shared among the user community
- Direct I/O for real-time users
- Primitive command language to avoid need for large internal state
- Minimum usable system <16 Kwords
- Modular software to correspond to modular hardware configurations
Instruction-Set Processor (ISP)
- Support user languages by data-types and special operations
Scientific (i.e., FORTRAN) Þ
integers, reals, Boolean
List processing (i.e., LISP) Þ
addresses, characters
Support recursive and reentrant programming Þ
stack mechanism
- Support operating systems
Effective as machine language Þ
Booleans, addresses, characters, I/O Operating system is an extension of hardware via defined operating codes
- Word length would be 36 bits (compatible with DEC's computers)
- Large (1/4 million 36-bit words = 1 million 9-bit bytes) address
- Require minimal hardware Þ
simple
- General-register based (design decision) with completely general use
- Easy to use and remember machine language
Orthogonality of addressing (accessing) and operators
Completeness of operators
Direct (not base + displacement) addressing
Few exceptional instructions
- 2's complement arithmetic (multiple precision arithmetic)
PMS Structure
- Maximum modularity so that users could easily configure any system
- Asynchronous operation - system must handle evolving technology
- Multiprocessors for incremental and increased performance (2-4 in design)
- No Pios (IBM channels), use simple programmed I/O with interrupts and direct-memory access for high-speed data transmission
Implementation
- Asynchronous logic and buses for speed in light of uncertain logic and memory speed
- All state accessible to field service personnel via lights
- Use DEC (10 MHz versus 5 MHz) circuit/logic technology (manpower constraint)
- Buildable without microprogramming (no fast, read-only memories in 1963)
Organizational/Marketplace
- Add to high end of DEC's computers
- Use minimal resources, while supporting DEC's minicomputer efforts
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