VAX

DEC VAX

VAX-11/780
Manufacturer	Digital Equipment Corporation
Byte size	8 bits (octet)
Address bus size	32 bits
Peripheral bus	Unibus, Massbus, Q-Bus, XMI, VAXBI
Architecture	CISC, virtual memory
Operating systems	VAX/VMS, Ultrix, BSD UNIX, VAXELN

VAX was an instruction set architecture (ISA), developed by Digital Equipment Corporation (DEC) in the mid-1970s. The VAX-11/780, introduced on October 25, 1977, was the first of a range of popular and influential computers implementing that architecture.

A 32-bit complex instruction set computer based on DEC's earlier PDP-11, VAX ("virtual address extension") was designed to extend or replace DEC's various PDP ISAs. The VAX architecture's primary features were virtual addressing (for example demand paged virtual memory) and its orthogonal instruction set.

VAX has been perceived as the quintessential CISC ISA, with its very large number of assembly-language-programmer-friendly addressing modes and machine instructions, highly orthogonal architecture, and instructions for complex operations such as queue insertion or deletion and polynomial evaluation.^[1]

Name

VAX 8350 front view with cover removed

The name "VAX" originated as an acronym for virtual address extension, both because the VAX was seen as a 32-bit extension of the older 16-bit PDP-11 and because it was (after Prime Computer) an early adopter of virtual memory to manage this larger address space. Early versions of the VAX processor implemented a "compatibility mode" that emulated many of the PDP-11's instructions, and were in fact called VAX-11 to highlight this compatibility and the fact that VAX-11 was an outgrowth of the PDP-11 family. Later versions offloaded the compatibility mode and some of the less used CISC instructions to emulation in the operating system software.

Instruction set

The VAX instruction set was designed to be powerful and orthogonal. When it was introduced, many programs were written in assembly language, so having a "programmer-friendly" instruction set was important. In time, as more programs were written in higher-level language, the instruction set became less visible, and the only ones much concerned about it were compiler writers.

One unusual aspect of the VAX instruction set is the presence of register masks at the start of each subprogram. These are arbitrary bit patterns that specify, when control is passed to the subprogram, which registers are to be preserved. Since register masks are a form of data embedded within the executable code, they can make linear parsing of the machine code difficult. This can complicate optimization techniques that are applied on machine code.^[2]

Operating systems

The "native" VAX operating system is DEC's VAX/VMS (renamed to OpenVMS in 1991 or early 1992 when it was ported to Alpha, modified to comply with POSIX standards, and "branded" as compliant with XPG4 by the X/Open consortium^[3]). The VAX architecture and VMS operating system were "engineered concurrently" to take maximum advantage of each other, as was the initial implementation of the VAXcluster facility. Other VAX operating systems have included various releases of BSD UNIX up to 4.3BSD, Ultrix-32, VAXELN and Xinu. More recently, NetBSD and OpenBSD support various VAX models and some work has been done on porting Linux to the VAX architecture.

History

K 1840, VAX-11/780 clone, 1988, Technical Collections Dresden

The first VAX model sold was the VAX-11/780, which was introduced on October 25, 1977 at the Digital Equipment Corporation's Annual Meeting of Shareholders.^[4] Bill Strecker, C. Gordon Bell's doctoral student at Carnegie Mellon University, was responsible for the architecture.^[5] Many different models with different prices, performance levels, and capacities were subsequently created. VAX superminicomputers were very popular in the early 1980s.

DEC VAX 11/780-5 computer.

For a while the VAX-11/780 was used as a standard in CPU benchmarks. It was initially described as a one-MIPS machine, because its performance was equivalent to an IBM System/360 that ran at one MIPS, and the System/360 implementations had previously been de facto performance standards. The actual number of instructions executed in 1 second was about 500,000, which led to complaints of marketing exaggeration. The result was the definition of a "VAX MIPS," the speed of a VAX-11/780; a computer performing at 27 VAX MIPS would run the same program roughly 27 times faster than the VAX-11/780. Within the Digital community the term VUP (VAX Unit of Performance) was the more common term, because MIPS do not compare well across different architectures. The related term cluster VUPs was informally used to describe the aggregate performance of a VAXcluster. (The performance of the VAX-11/780 still serves as the baseline metric in the BRL-CAD Benchmark, a performance analysis suite included in the BRL-CAD solid modeling software distribution.) The VAX-11/780 included a subordinate stand-alone LSI-11 computer that performed microcode load, booting, and diagnostic functions for the parent computer. This was dropped from subsequent VAX models. Enterprising VAX-11/780 users could therefore run three different Digital Equipment Corporation operating systems: VMS on the VAX processor, and either RSX-11M or RT-11 on the LSI-11.

The VAX went through many different implementations. The original VAX 11/780 was implemented in TTL and filled a four-by-five-foot cabinet^[6] with a single CPU. CPU implementations that consisted of multiple ECL gate array or macrocell array chips included the VAX 8600 and 8800 superminis and finally the VAX 9000 mainframe class machines. CPU implementations that consisted of multiple MOSFET custom chips included the 8100 and 8200 class machines. The VAX 11-730 and 725 low end machines were built using bit-slice components.

VAX 11/750

The MicroVAX I represented a major transition within the VAX family. At the time of its design, it was not yet possible to implement the full VAX architecture as a single VLSI chip (or even a few VLSI chips as was later done with the V-11 CPU of the VAX 8200/8300). Instead, the MicroVAX I was the first VAX implementation to move some of the more complex VAX instructions (such as the packed decimal and related opcodes) into emulation software. This partitioning substantially reduced the amount of microcode required and was referred to as the "MicroVAX" architecture. In the MicroVAX I, the ALU and registers were implemented as a single gate-array chip while the rest of the machine control was conventional logic.

A full VLSI (microprocessor) implementation of the MicroVAX architecture arrived with the MicroVAX II's 78032 (or DC333) CPU and 78132 (DC335) FPU. The 78032 was the first microprocessor with an on-board memory management unit^[7] The MicroVAX II was based on a single, quad-sized processor board which carried the processor chips and ran the MicroVMS or Ultrix-32 operating systems. The machine featured 1 MB of on-board memory and a Q22-bus interface with DMA transfers. The MicroVAX II was succeeded by many further MicroVAX models with much improved performance and memory.

Further VLSI VAX processors followed in the form of the V-11, CVAX, CVAX SOC ("System On Chip", a single-chip CVAX), Rigel, Mariah and NVAX implementations. The VAX microprocessors extended the architecture to inexpensive workstations and later also supplanted the high-end VAX models. This wide range of platforms (mainframe to workstation) using one architecture was unique in the computer industry at that time. Sundry graphics were etched onto the CVAX microprocessor die. The phrase CVAX... when you care enough to steal the very best was etched in broken Russian as a play on a Hallmark Cards slogan, intended as a message to Soviet engineers who were known to be both purloining DEC computers for military applications and reverse engineering their chip design.^[8][9]

In DEC's product offerings, the VAX architecture was eventually superseded by RISC technology. In 1989 DEC introduced a range of workstations and servers that ran Ultrix, the DECstation and DECsystem respectively, based on processors that implemented the MIPS architecture. In 1992 DEC introduced their own RISC instruction set architecture, the Alpha AXP (later renamed Alpha), and their own Alpha-based microprocessor, the DECchip 21064, a high performance 64-bit design capable of running OpenVMS.

In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year.^[10] By 2005 all manufacturing of VAX computers had ceased, but old systems remain in widespread use.

The Stromasys CHARON-VAX, SIMH and Vere Technologies vtVAX software-based VAX emulators remain available.

Processor architecture

MicroVAX 3600 (left) with printer (right)

DEC VAX registers

^31 . . . ^23 . . . ^15 ^14 ^13 ^12 ^11 ^10 ^09 ^08 ^07 ^06 ^05 ^04 ^03 ^02 ^01 ^00 (bit position) General registers R0 Register 0 R1 Register 1 R2 Register 2 R3 Register 3 R4 Register 4 R5 Register 5 R6 Register 6 R7 Register 7 R8 Register 8 R9 Register 9 R10 Register 10 R11 Register 11 R12 / AP Register 12 / Argument Pointer R13 / FP Register 13 / Frame Pointer R14 / SP Register 14 / Stack Pointer R15 / PC Register 15 / Program Counter Status flags N Z V C Condition Code Register

VAX

Designer	Digital Equipment Corporation
Bits	32 bits
Introduced	1977
Design	CISC
Type	Memory-Memory
Encoding	Variable (1 to 56 bytes)
Branching	Condition code
Endianness	Little
Extensions	PDP-11 compatibility mode, VAXvector [11]
Open	No
Registers
General purpose	16[12]
Floating point	uses the GPRs

Virtual memory map

The VAX virtual memory is divided into four sections, each of which is one gigabyte (In the context of addressing, 2³⁰ bytes) in size:

Section	Address Range
P0	0x00000000 - 0x3fffffff
P1	0x40000000 - 0x7fffffff
S0	0x80000000 - 0xbfffffff
S1	0xc0000000 - 0xffffffff

For VMS, P0 was used for user process space, P1 for process stack, S0 for the operating system, and S1 was reserved.

Privilege modes

The VAX has four hardware implemented privilege modes:

No.	Mode	VMS Usage	Notes
0	Kernel	OS Kernel	Highest Privilege Level
1	Executive	File System
2	Supervisor	Shell (DCL)
3	User	Normal Programs	Lowest Privilege Level

Processor status register

CM	TP	MBZ	FD	IS	cmod	pmod	MBZ	IPL	MBZ	DV	FU	IV	T	N	Z	V	C
31	30	29	27	26	25	23	21	20	15	7	6	5	4	3	2	1	0

Bits	Meaning
31	PDP-11 compatibility mode
30	trace pending
29:28	MBZ (must be zero)
27	first part done (interrupted instruction)
26	interrupt stack
25:24	current privilege mode
23:22	previous privilege mode
21	MBZ (must be zero)
20:16	IPL (interrupt priority level)
15:8	MBZ (must be zero)
7	decimal overflow trap enable
6	floating-point underflow trap enable
5	integer overflow trap enable
4	trace
3	negative
2	zero
1	overflow
0	carry

Addressing modes

The SPEC-1 VAX, a VAX 11/780 used for benchmarking, showing internals

The VAX supports many addressing modes: literal, register, postincrement, predecrement, register deferred, postincrement deferred, predecrement deferred, displacement (byte, word, long), displacement (byte, word, long) deferred; also indexed, which may be combined with many of these. An "immediate" mode is synonymous with program counter (PC) postincrement, and many addressing modes could use the program counter (which is also R15) instead of other registers. This provided for easy generation of position-independent code through "PC-relative" addressing. The VAX also has some "load effective address" instructions, which do not access memory but compute the address that should be used.

VAX-based systems

The first VAX-based system was the VAX-11/780, a member of the VAX-11 family. The high-end VAX 8600 replaced the VAX-11/780 in October 1984 and was joined by the entry-level MicroVAX minicomputers and the VAXstation workstations in the mid-1980s. The MicroVAX was superseded by the VAX 4000, the VAX 8000 was superseded by the VAX 6000 in the late 1980s and the mainframe-class VAX 9000 was introduced. In the early 1990s, the fault-tolerant VAXft was introduced, as were the Alpha compatible VAX 7000/10000. A variant of various VAX-based systems were sold as the VAXserver.

Cancelled systems

Cancelled systems include the "BVAX", a high-end ECL-based VAX, and two other ECL-based VAX models: "Argonaut" and "Raven".^[13] A VAX known as "Gemini" was also cancelled, which was a fall-back in case the LSI-based Scorpio failed. It never shipped.

Clones

A number of VAX clones, both authorized and unauthorized, were produced. Examples include:

• Systime Ltd. of the United Kingdom produced clones of early VAX models such as the Systime 8750 (equivalent to the VAX 11/750).^[14]
• Norden Systems produced the ruggedized, Military-specification MIL VAX series.^[3]
• The Hungarian Central Research Institute for Physics (KFKI) produced a series of clones of early VAX models, the TPA-11/540, 560 and 580.^[15]
• The SM 52/12^[16] from Czechoslovakia, developed at VUVT Žilina (today Slovakia) and produced from 1986 at ZVT Banská Bystrica (today Slovakia).
• The East German VEB Robotron K 1840 (SM 1710) was a clone of the VAX-11/780 and Robotron K 1820 (SM 1720) was a copy of the MicroVAX II.
• The SM-1700 was a Soviet clone of the VAX-11/730, SM-1702 was a clone of MicroVAX II and SM-1705 was a clone of VAX-11/785.^[17]

References

1. Payne, Mary; Bhandarkar, Dileep (1980). "VAX floating point: a solid foundation for numerical computation". SIGARCH Comput. Archit. News (ACM) 8 (4): pp 22–33. doi:10.1145/641845.641849. ISSN 0163-5964.
2. Clinton F. Goss (August 2013) [First published June 1986]. "Machine Code Optimization — Improving Executable Object Code" (PDF) (Ph.D. dissertation). Computer Science Department Technical Report #246. Courant Institute, New York University. arXiv:1308.4815. Retrieved 22 Aug 2013. Lay summary.
3. Rainville, Jim; Howard, Karen, eds. (1997). "VAX/VMS at 20". Digital Equipment Corporation. Retrieved 2009-07-27.
4. "VAX 11/780, The First VAX System (October 1977)".
5. Slater, Robert (1987). Portraits in Silicon. MIT Press. p. 213. ISBN 978-0-262-69131-4.
6. "VAX 11/780 Computer – CPU". Computer History Museum. Retrieved 2012-10-24.
7. The Computer History Simulation Project: MicroVAX II (1985)
8. micro.magnet.fsu.edu, Steal the best, retrieved 30 January 2008. The Russian phrase was: СВАКС... Когда вы забатите довольно воровать настоящий лучший
9. The Computer History Simulation Project: CVAX (1987), retrieved 30 January 2008
10. "VAX Systems: A letter from Jesse Lipcon". Archived from the original on 2000-08-15.
11. "VAX MACRO and Instruction Set Reference Manual". OpenVMS documentation. April 2001.
12. VAX Architecture & Assembly Reference (PDF). Department of Computer Science, Rochester Institute of Technology. Retrieved 2012-09-14.
13. Mark Smotherman (2008-07-19). "Who are the Computer Architects?". Retrieved 2008-09-30.
14. "RAL Informatics Report 1984-85". Retrieved 2007-10-15.
15. "The TPA story". Retrieved 2007-10-15.
16. Dujnic, J.; Fristacky, N.; Molnar, L.; Plander, I.; Rovan, B.: On the history of computer science, computer engineering, and computer technology development in Slovakia. IEEE Annals of the History of Computing, vol. 21, no. 3, pp. 38-48, July-Sept. 1999
17. Laimutis Telksnys; Antanas Zilinskas (July 1999). "Computers in Lithuania" (PDF). IEEE Annals of the History of Computing 21 (3): 31–37.

External links

• HP: VAX Systems
• DEC Microprocessors
• SimH VAX Open source emulator that supports VAX architecture