VAX Architecture

Memory - The VAX memory consists of 8-bit bytes. All addresses used are byte addresses. Two consecutive bytes form a word, Four bytes form a longword, eight bytes form a quadword, sixteen bytes form a octaword. All VAX programs operate in a virtual address space of 232 bytes , One half is called system space, other half process space.

Registers – There are 16 general purpose registers (GPRs) , 32 bits each, named as R0 to R15, PC (R15), SP (R14), Frame Pointer FP ( R13), Argument Pointer AP (R12) ,Others available for general use. There is a Process status longword (PSL) – for flags.
Data Formats - Integers are stored as binary numbers in byte, word, longword, quadword, octaword. 2’s complement notation is used for storing negative numbers. Characters are stored as 8-bit ASCII codes. Four different floating-point data formats are also available.

Instruction Formats - VAX architecture uses variable-length instruction formats – op code 1 or 2 bytes, maximum of 6 operand specifiers depending on type of instruction. Tabak – Advanced Microprocessors (2nd edition) McGraw-Hill, 1995, gives more information.

Addressing Modes - VAX provides a large number of addressing modes. They are Register mode, register deferred mode, autoincrement, autodecrement, base relative, program-counter relative, indexed, indirect, and immediate.

Instruction Set – Instructions are symmetric with respect to data type - Uses prefix – type of operation, suffix – type of operands, a modifier – number of operands. For example, ADDW2 - add, word length, 2 operands, MULL3 - multiply, longwords, 3 operands CVTCL - conversion from word to longword. VAX also provides instructions to load and store multiple registers.

Input and Output - Uses I/O device controllers. Device control registers are mapped to separate I/O space. Software routines and memory management routines are used for input/output operations.