VAX Architecture
CPU
R0
ALU
R1
.
.
Control Unit
R12 (AP)
R13 (FP)
Local Storage
R14 (SP)
R15 (PC)
PSL
Memory Increment Bus
Memory
BI adaptor
BI Bus BI adaptor
Input/Output
devices
Structure of a VAX 8810
BI Bus
Input/Output
devices
 Registers provide a local high-speed storage for the processor.
 There are 16 registers (all 32-bit) in a VAX CPU
 Six of these registers (R6 - R11) are general purpose registers.
 R0 - R5 are also general purpose registers, however, they
should be used with care because they are also implicitly
changed by some VAX instructions.
 R12 - R14 are special purpose registers
R12 - Argument Pointer
R13 - Frame Pointer
R14 - Stack Pointer
 Register R15 (Program Counter or PC)
program counter:
Holds the address of the next instruction to be executed. PC is
updated by the number of bytes of the instruction.
 the address of an instruction is the address of its opcode.
Process Status Register:
Stores status information which describe the current state of the
program in execution. Most important are condition codes.
Condition codes are flags that provide information about the
result obtained from instruction execution.
Codes include
- Z flag: Set if the result is 0
- V flag: Set if the result is out of range
- N flag: Set if the result is negative
- C flag: Set if there was a carry/borrow
In VAX Architecture this register is called the process status
longword
31
0
Not accessible
0000000
15
PSW
DV FU IV T N Z V C
0
 T - Trace trap flag; it is used by the VAX Symbolic
Debugger.
 IV - Integer Overflow Trap
 FU - Floating-point Underflow Trap
 DI - Decimal Overflow Trap.
When a trap bit for one of these errors is set, the VAX
hardware causes a termination of execution when that error is
detected, otherwise the error is ignored.
** Traps are exceptions that occur at the end of an
instruction.
** An exception (Software Interrupt) is problem in an
instruction that causes an interruption in the normal flow
of execution.
Register Usage in the VAX architecture:
 There are 16 registers
 All registers are 32-bit
 Registers are numbered R0 - R15.
 R0 - R11 used in programs in byte (8 bit),
Word (16 bit), or Longword (32 bits)
R0 - R5 Should be used with caution, they are
used implicitly by some instructions.
R12 - AP
R13 - FP
R14 - SP
R15 - PC (Not accessible by programmer)
Memory Organization in VAX Architecture:
 Address space:
Is the set of all addresses. The number of bits on the
address bus determine the address space of the
machine
ex.
32-bit address bus can address up to 4GByte of memory
Digression:
210 = 1K
220 = 1M
230 = 1G
 Virtual Memory:
The idea of executing a program while only a portion of
the program resides in main memory. (make believe that
there is more physical memory available). This is done by
breaking the program into equal size blocks (pages) and
bringing it into main memory on demand. The operating
system handles this mapping between the VM and physical
memory.
The VAX address bus is 32-bit (every address is a 32 bit
unsigned integer)
 address space is 4GB
1/2 is used by users programs
1/2 is used by the system
Addresses are virtual addresses, not physical
 Memory is organized in bytes (byte –addressable)
 If a data item is more than one byte, contiguous memory cells
will be allocated for it. VAX uses the little-endian
technique. (least-significant byte first)
address of data b0
byte1
byte0
b1
Data Types Supported by VAX
BYTE
WORD
LONGWORD
QUADWORD
OCTAWORD
8 bits
16 bits
32 bits
4 words (8 bytes)
8 words (16 bytes)
Instruction Format:
In the source file....
[Label:] Opcode Operand_Specifier_1,
Operand_Specifier_2, ..... [;Comment]
[ ] is an optional field.
ex. ADDL2 R1, R2
memory layout of instructions
opcode
op
spec.
.
.
 The first Byte of each instruction is an opcode (operation
code).
 The address of an instruction is the address of the byte
where the opcode is stored.
 Opcodes in the VAX are one byte (most of them). Very
few opcodes are two bytes.
Opcodes tell the CPU the following information:
1- The operation
ex.
1000 0000 in VAX is an ADDB2 instruction
2- Number of operands
ex.
1000 0000 requires two operands
ADDB2 R1, R2
R1,and R2 are operands
3- Data type upon which it will operate
ex. ADDB2 operates on byte data.
 Data type is specified using letters
B Byte
W Word
L Longword
Q Quadword
O Octaword
D, F, H, G data type for floating-point data...
 Symbols are names of variables, subprograms, and they
label instructions.
 Symbols are made up of letters, digits, and underscore.
Must start with a letter and must be less than 31 chars.
 A Label is a symbolic name for an address. If an
instruction is started with a label, then it can be
addressed by this label in the program. We use a label to
allow for branching, and naming memory location.
ex. Label: ADDB2 R1, R2
The Label here is the address of this instruction.
label
opcode
rest
of inst.
- Not every instruction needs a label.
 Operands are either in memory or in registers.
Programmers must specify the ways in which the
instruction will find them. The different ways in
specifying the location of the operands are called
ADDRESSING MODES.
Effective Address:
Is the address where the operand is stored (the address
where data to be operated on is located)
In the VAX there are 20 different addressing modes.
 There are 3 types of statements that can appear in a
source file in the MACRO assembly language
Instructions.
These are executable instructions. They get translated into
machine code by the assembler and executed at run time.
ex.
ADDL2, DIVB3, LOCC, MOVC5 and all the
instructions in the instruction set.
Pseudo-instructions (Macros)
These are names that get replaced by the assembler by a
sequence of other instructions that do a certain task. (We
will talk about Macros in Chap. 11)
Directives.
These are instructions (NOT EXEUTABLE) to the
assembler to perform various bookkeeping tasks, storage
reservations, and other control functions.
They start with "." in the front of the instruction.
ex. .LONG, .BYTE
- Directives are not translated to machine code.
- They resemble declarations in HLLs.
Storage (memory) Reservation in Assembly:
There are two types:
1- Reserve blocks with an initial value of 0
ex. int x,y; (in C)
2- Reserve blocks with initial values
ex. int x = 3;
Reserving initialized memory
.BYTE
.WORD
.QUAD
.OCTA
.ASCIZ and more
.LONG
.ADDRESS
.ASCII
-Each one of the above directives takes a list of
arguments.
-The argument of these directives is the value to
which the memory is initialized
Ex.
X: .BYTE 5
This tells the assembler to allocate a byte of memory and
initialize it to 5.
 The assember uses two's complement to represent
negative numbers.
X: 0000 0101
ex.
W: .LONG 7
W:
0000 0111
0000 0000
0000 0000
0000 0000
ex.
Fernando: .WORD -3
Fernando
1111 1101
1111 1111
ex.
Alpha: .BYTE 3, 5, 76
Multiple blocks can be allocated with one directive
ex.
X: .BYTE 5[3]
of blocks}
{number between [] is the number
Allocating memory for character string
(.ASCII, .ASCIZ)
 Use ASCII code. Each char occupies a byte.
 Must use delimiters like / , " , {, ...
 Any character can be used as a delimiter so long as this char
does not appear in the character string.
 The .ASCII directive allocates a byte for each char in the
string.
 .ASCIZ is the same as .ASCII except it adds an extra (all
zeros) byte at the end of the string. A zero byte is the NULL
character in ASCII.
ex.
message: .ASCII /Hello World/
MSG: .ASCIZ "this is fun"
User Defined Symbols
 The user can define symbols explicitly by equating them to
the desired value. (Direct Assignment)
Symbol = Expression
*** This is not an assignment statement as in HLL.
ex.
CR = 13
LF = 10
This is like a DEFINE directive in C
 The assembler replaces each occurrence of these symbols by
their values during assembly.
ex.
MAX = 10
B : .BYTE 61
Control characters are ASCII codes that are not printable.
Example: a carriage return or line feed.
To include control characters in the program we must
use their ASCII representation (code) in angle brackets
ex.
linefeed <10>
Carriage return <13>
ex.
T1: .ASCIZ /Hello World/
T2: .ASCII /Firas is/ <13><10> /human/
T3: .ASCII /So what/
Reserving Uninitialized memory
.BLKx
where x = B, W, L, O, Q, A, and more
ex.
.BLKB 3
.BLKW 1
 The argument specifies the number of blocks to be
reserved.
 The assembler allocates the blocks and initializes them
to zeros.
In the above example .BLKB 3 tells the assembler to
allocate 3 bytes
 Memory allocation should be before or after the
executable instructions.
 You can have a memory allocation directive within
executable instructions as long as you have a way to
jump over these blocks. (we will talk more on this later)
** All constant values in the assembly program are
decimal numbers unless explicitly specified
otherwise.
 To represent constants with number systems other than
decimal a special operator must be used. The operator is
followed by one of the symbols (B, X, O, C, A).
B
X
O
C
A
for binary
for hexadecimal
for octal
used to form the one's complement of its argument
for ASCII code
ex. ^B10110011
^XF76E
^A/F/ (ASCII value of the char F
Beginning and Ending Programs:
.ENTRY Directive
- Establishes an entry point
- Entry point is a place where execution starts.
.ENTRY symbol, Mask
ex.
.ENTRY main, ^M<IV>
 The next instruction should be an executable instruction.
 The symbol in the directive is the address of the first
instruction of the program.
.END Directive
 Tells the assembler that this is the end of the module
that is being translated.
 Each file must have a .END directive at the end of the
file.
.END
transfer_address
 Transfer address is a symbol that specifies where
execution of the program is to begin.
 Transfer address should be the same as the symbol in
the .ENTRY directive.
$EXIT_S Macro
 It terminates the execution of the program
 It is a macro that gets replaced by a sequence of
instructions
ex.
ALPHA: .LONG 224
BETA: .BLKL 1
.ENTRY EXAMPLE, ^<IV>
MULL3 #3,Alpha,R6
SUBL3 #20,R6,Beta
$EXIT_S
.END EXAMPLE
Instructions:
Arithmetic
operation
type
ADD
SUB
MUL
DIV
B,W,L
B,W,L
B,W,L
B,W,L
ADDx2
op1 + op2  op2
ex.
x = x+y
ADDL2 Y,X
# of operand
2,3
2,3
2,3
2,3
op1,op2
ADDx3 op1,op2,op3
op1+op2  op3
the same applies to MUL instruction
SUB
SUBx2 op1, op2
op2 – op1  op2
SUBx3 op1, op2, op3
op2 – op1 op3
DIV
DIVx2 op1, op2
op2/op1  op2
DIVx3 op1, op2, op3
op2/op1  op3
Ex.
Compute
R7 R44 + R3 * R6
Special Purpose Instructions:
MOVx
Source, Dest.
X = B, W, L, Q, O
Source  Dest.
Ex.
MOVL R2, R4
INCx operand
X = B, W, L
Operand +1  operand
DECx operand
X = B, W, L
Operand – 1  operand
MNEGx source, dest.
X = B, W, L, Q, O
(- source)  dest.
Ex.
R8 =
CLRL
0000 9AFF
R8
CLRB
R8
CLRW
R8
INCB
R8
INCW
Ex.
R8
FFED 01A2 R4
1234 00D7
R6
0002 1044
R10
Alpha
10
00
1A
40
Beta
04
F1
FF
FF
Gamma 3B
0A
48
27
0002 1044
0002 1045
0002 1046
0002 104F
ADDW2 R4, R10
ADDB3 ALPHA,BETA, R6