HP OpenVMS Systems Documentation

VAX MACRO and Instruction Set Reference Manual

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5.1.8 Literal Mode

In literal mode, the value of the literal is stored in the addressing mode byte.

Formats

#literal
S^#literal

Parameters

literal
An expression, an integer constant, or a floating-point constant. The literal must fit in the short literal form. That is, integers must be in the range 0 to 63 and floating-point constants must be one of the 64 values listed in Table 5-2 and Table 5-3. Floating-point short literals are stored with a 3-bit exponent and a 3-bit fraction. Table 5-2 and Table 5-3 also show the value of the exponent and the fraction for each literal. See Section 8.7.8 for information on the format of short literals.

**Table 5-2 Floating-Point Literals Expressed as Decimal Numbers**
Exponent	0	1	2	3	4	5	6	7
0	0.5	0.5625	0.625	0.6875	0.75	0.8125	0.875	0.9375
1	1.0	1.125	1.25	1.37	1.5	1.625	1.75	1.875
2	2.0	2.25	2.5	2.75	3.0	3.25	3.5	3.75
3	4.0	4.5	5.0	5.5	6.0	6.5	7.0	7.5
4	8.0	9.0	10.0	11.0	12.0	13.0	14.0	15.0
5	16.0	18.0	20.0	22.0	24.0	26.0	28.0	30.0
6	32.0	36.0	40.0	44.0	48.0	52.0	56.0	60.0
7	64.0	72.0	80.0	88.0	96.0	104.0	112.0	120.0

**Table 5-3 Floating-Point Literals Expressed as Rational Numbers**
Exponent	0	1	2	3	4	5	6	7
0	1/2	9/16	5/8	11/16	3/4	13/16	7/8	15/16
1	1	1-1/8	1-1/4	1-3/8	1-1/2	1-5/8	1-3/4	1-7/8
2	2	2-1/4	2-1/2	2-3/4	3	3-1/4	3-1/2	3-3/4
3	4	4-1/2	5	5-1/2	6	6-1/2	7	7-1/2
4	8	9	10	11	12	13	14	15
5	16	18	20	22	24	26	28	30
6	32	36	40	44	48	52	56	60
7	64	72	80	88	96	104	112	120

Example

MOVL    #1,R0           ; R0 is set to 1; the 1 is stored
                        ;   in the instruction as a short
                        ;   literal.

MOVB    S^#CR,R1        ; The low byte of R1 is set
                        ;   to the value CR.
                        ;   CR is stored in the instruction
                        ;   as a short literal.
                        ;   If CR is not in range 0-63,
                        ;   the linker produces a
                        ;   truncation error.

MOVF    #0.625,R6       ; R6 is set to the floating-point
                        ;   value 0.625; it is stored
                        ;   in the floating-point short
                        ;   literal form.

Notes

When you use the #literal format, the assembler chooses whether to use literal mode or immediate mode (see Section 5.2.4). The assembler uses immediate mode if any of the following conditions is satisfied:
- The value of the literal does not fit in the short literal form.
- The literal is a relocatable or external expression (see Section 3.5).
- The literal is an expression that contains undefined symbols.
The difference between immediate mode and literal mode is the amount of storage that it takes to store the literal in the instruction.
The S^#literal format forces the assembler to use literal mode.

5.2 Program Counter Modes

The program counter (PC) modes use the PC for a general register. Following are the five program counter modes:

Relative
Relative deferred
Absolute
Immediate
General

In Section 8.8, Table 8-6 is a summary of PC addressing.

5.2.1 Relative Mode

In relative mode, the address specified is the address of the operand. The assembler stores the address as a displacement from the PC.

Relative mode can be used with index mode (see Section 5.3).

Format

address

Parameters

address
An expression specifying an address; the expression can be preceded by one of the following displacement length specifiers, which indicate the number of bytes needed to store the displacement.

Displacement Length Specifier	Meaning
B^	Displacement requires 1 byte.
W^	Displacement requires one word (2 bytes).
L^	Displacement requires one longword (4 bytes).

If no displacement length specifier precedes the address expression, and the value of the expression is known, the assembler chooses the smallest number of bytes (1, 2, or 4) needed to store the displacement. If no length specifier precedes the address expression, and the value of the expression is unknown, the assembler uses the default displacement length (see the description of .DEFAULT in Chapter 6). If the address expression is either defined later in the program or defined in another program section, the assembler considers the value unknown.

Example

MOVL    LABEL,R1        ; Get longword at LABEL; the
                        ;   assembler uses default
                        ;   displacement unless LABEL was
                        ;   previously defined in this
                        ;   section

CMPL    W^<DATA+4>,R10  ; Compare R10 with longword at
                        ;   address DATA+4; CMPL
                        ;   uses a word displacement

5.2.2 Relative Deferred Mode

In relative deferred mode, the address specified is the address of the operand address (a pointer to the operand). The assembler stores the address specified as a displacement from the PC.

Relative deferred mode can be used with index mode (see Section 5.3).

Format

@address

Parameters

address
An expression specifying an address; the expression can be preceded by one of the following displacement length specifiers, which indicate the number of bytes needed to store the displacement:

Displacement Length Specifier	Meaning
B^	Displacement requires 1 byte.
W^	Displacement requires one word (2 bytes).
L^	Displacement requires one longword (4 bytes).

Example

CLRL    @W^PNTR         ; Clear longword pointed to by
                        ;   longword at PNTR; the assembler
                        ;   uses a word displacement

INCB    @L^COUNTS+4     ; Increment byte pointed to by
                        ;   longword at COUNTS+4; assembler
                        ;   uses a longword displacement

5.2.3 Absolute Mode

In absolute mode, the address specified is the address of the operand. The address is stored as an absolute virtual address (compare relative mode, where the address is stored as a displacement from the PC).

Absolute mode can be used with index mode (see Section 5.3).

Format

@#address

Parameters

address
An expression specifying an address.

Example

CLRL    @#^X1100        ; Clear the contents of location 1100(hex)

CLRB    @#ACCOUNT       ; Clear the contents of location
                        ;   ACCOUNT; the address is stored
                        ;   absolutely, not as a displacement

CALLS   #3,@#SYS$FAO    ; Call the procedure SYS$FAO with
                        ;   three arguments on the stack

5.2.4 Immediate Mode

In immediate mode, the literal specified is the operand.

Formats

#literal
I^#literal

Parameters

literal
An expression, an integer constant, or a floating-point constant.

Example

MOVL        #1000,R0        ; R0 is set to 1000; the operand 1000
                            ;   is stored in a longword

MOVB        #BAR,R1         ; The low byte of R1 is set
                            ;   to the value of BAR

MOVF        #0.1,R6         ; R6 is set to the floating-point
                            ;   value 0.1; it is stored
                            ;   as a 4-byte floating-point
                            ;   value (it cannot be
                            ;   represented as a short literal)

ADDL2        I^#5,R0        ; The 5 is stored in a longword
                            ;   because the I^ forces the
                            ;   assembler to use immediate mode

MOVG        #0.2,R6         ; The value 0.2 is converted
                            ;   to its G_FLOATING representation

MOVG        #PI,R6          ; The value contained in PI is
                            ;   moved to R6; no conversion is
                            ;   performed

Notes

When you use the #literal format, the assembler chooses whether to use literal mode ( Section 5.1.8) or immediate mode. If the literal is an integer from 0 to 63 or a floating-point constant that fits in the short literal form, the assembler uses literal mode. If the literal is an expression, the assembler uses literal mode if all the following conditions are met:
- The expression is absolute.
- The expression contains no undefined symbols.
- The value of the expression fits in the short literal form.
In all other cases, the assembler uses immediate mode.
The difference between immediate mode and literal mode is the amount of storage required to store the literal in the instruction. The assembler stores an immediate mode literal in a byte, word, or longword depending on the operand data type.
The I^#literal format forces the assembler to use immediate mode.
You can specify floating-point numbers two ways: as a numeric value or as a symbol name. The assembler handles these values in different ways, as follows:
- Numeric values are converted to the appropriate internal floating-point representation.
- Symbols are not converted. The assembler assumes that the values have already been converted to internal floating-point representation.
Once the assembler obtains the value, it tries to convert the internal representation of the value to a short floating literal. If conversion fails, the assembler uses immediate mode; if conversion succeeds, the assembler uses short floating literal mode.

5.2.5 General Mode

In general mode, the address you specify is the address of the operand. The linker converts the addressing mode to either relative or absolute mode. If the address is relocatable, the linker converts general mode to relative mode. If the address is absolute, the linker converts general mode to absolute mode. You should use general mode to write position-independent code when you do not know whether the address is relocatable or absolute. A general addressing mode operand requires 5 bytes of storage.

You can use general mode with index mode (see Section 5.3).

Format

G^address

Parameters

address
An expression specifying an address.

Example

CLRL    G^LABEL_1           ; Clears the longword at LABEL_1
                            ;   If LABEL_1 is defined as
                            ;   absolute then general mode is
                            ;   converted to absolute
                            ;   mode; if it is defined as
                            ;   relocatable, then general mode is
                            ;   converted to relative mode

CALLS   #5,G^SYS$SERVICE    ; Calls procedure SYS$SERVICE
                            ;   with 5 arguments on stack

5.3 Index Mode

Index mode is a general register mode that can be used only in combination with another mode (the base mode). The base mode can be any addressing mode except register, immediate, literal, index, or branch. The assembler first evaluates the base mode to get the base address. To get the operand address, the assembler multiplies the contents of the index register by the number of bytes of the operand data type, then adds the result to the base address.

Combining index mode with the other addressing modes produces the following addressing modes:

Register deferred index
Autoincrement index
Autoincrement deferred index
Autodecrement index
Displacement index
Displacement deferred index
Relative index
Relative deferred index
Absolute index
General index

The process of first evaluating the base mode and then adding the index register is the same for each of these modes.

Formats

base-mode[Rx]
base-mode[AP]
base-mode[FP]
base-mode[SP]

Parameters

base-mode
Any addressing mode except register, immediate, literal, index, or branch, specifying the base address.
x
A number in the range 0 to 12, specifying the index register.

Table 5-4 lists the formats of index mode addressing.

Example

;
; Register deferred index mode
;
OFFS=20                         ; Define OFFS
        MOVAB   BLIST,R9        ; Get address of BLIST
        MOVL    #OFFS,R1        ; Set up index register
        CLRB    (R9)[R1]        ; Clear byte whose address
                                ;   is the address of BLIST
                                ;   plus 20*1

        CLRQ    (R9)[R1]        ; Clear quadword whose
                                ;   address is the address
                                ;   of BLIST plus 20*8

        CLRO    (R9)[R1]        ; Clear octaword whose
                                ;   address is the address
                                ;   of BLIST plus 20*16
;
; Autoincrement index mode
;
        CLRW    (R9)+[R1]       ; Clear word whose address
                                ;   is address of BLIST plus
                                ;   20*2; R9 now contains
                                ;   address of BLIST+2
;
; Autoincrement deferred index mode
;
        MOVAL   POINT,R8        ; Get address of POINT
        MOVL    #30,R2          ; Set up index register
        CLRW    @(R8)+[R2]      ; Clear word whose address
                                ;   is 30*2 plus the address
                                ;   stored in POINT; R8 now
                                ;   contains 4 plus address of
                                ;   POINT
;
; Displacement deferred index mode
;
        MOVAL   ADDARR,R9       ; Get address of address array
        MOVL    #100,R1         ; Set up index register
        TSTF    @40(R9)[R1]     ; Test floating-point value
                                ;   whose address is 100*4 plus
                                ;   the address stored at
                                ;   (ADDARR+40)

**Table 5-4 Index Mode Addressing**
Mode	Format
Register Deferred Index ^1,2	(Rn)[Rx]
Autoincrement Index ^1,2	(Rn)+[Rx]
Autoincrement Deferred Index ^1,2	@(Rn)+[Rx]
Autodecrement Index ^1,2	-(Rn)[Rx]
Displacement Index ^1,2,3	dis(Rn)[Rx]
Displacement Deferred Index ^1,2,3	@dis(Rn)[Rx]
Relative Index ²	address[Rx]
Relative Deferred Index ²	@address[Rx]
Absolute Index ²	@#address[Rx]
General Index ²	G^address[Rx]

¹Rn---Any general register R0 to R12 or the AP, FP, or SP register.
²Rx---Any general register R0 to R12 or the AP, FP, or SP register. Rx cannot be the same register as Rn in the autoincrement index, autoincrement deferred index, and decrement index addressing modes.
³dis---An expression specifying a displacement.

Notes

If the base mode alters the contents of its register (autoincrement, autoincrement deferred, and autodecrement), the index mode cannot specify the same register.
The index register is added to the address after the base mode is completely evaluated. For example, in autoincrement deferred index mode, the base register contains the address of the operand address. The index register (times the length of the operand data type) is added to the operand address rather than to the address stored in the base register.

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