VAX-11

The Computer Architecture

VAX-11 Timeline

1978 VAX-11/780

1980 VAX-11/750

1982 VAX-11/730

1982 VAX-11/782

1984 VAX-11/725

1984 VAX-11/785

Floating-point representation

• F_floating – (single precision)

• D_floating – (double precision)

• G_floating – (double precision, extended range)

• H_floating – (quad precision)

F_floating-point representation

VAX-11 Floating Point Representations: "F_Floating" Structure (32 bit "longword"):

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Fraction (second part): bit 16 is the least significantSign Bit

Exponent Fraction (first part): bit 6 is the most significant

Language and the Machine

• CISC machine

• Large instruction set

• Only 17 registers

• 16 memory address modes

• Little-endian machine

Basic machine code

• Starts with one or two byte op-code– Contains four bits to determine address mode

• Followed by zero to five operand specifiers

Register Mode

• The one or two byte op-code is followed by (0101)2

• Those bits are followed by the four bit register address

• Example:– [One or two byte op-code]01010000

• This code would actually be reversed when stored because it is a little-endian machine

Microprogramming

• Early implementation of microprogramming

• Designed with ease of compilation in mind

• Result was complex instructions– Difficult to implement high-performance

applications

• Downfall was slowness of microprograms

• Here’s why….

Microinstruction Format

• 96 bits, 30 unique fields

15 13 12 0

VAK FEK SCK

31 30 29 26 25 24 23 22 20 19 18 17 16

ADS FS

47 46 43 42 41 35 34 32

63 58 57 55 54 51 50 48

RMX

79 78 77 76 72 71 70 69 66 65 64

95 92 91 88 87 85 84 82 81 80

AMX

ACF

IBC DK SHF BMX

DT BEN ALU SUB

MCT/CID SPO PCK

KMX SI/ACM QK SGN

EALU JMP

IEK MSC CCK EBMX SMX

Conclusion

• Superior system, for its time• Birth of the VUP• Technology eventually renders

obsolete