Updated: 11 December 1998

VAX MACRO and Instruction Set Reference Manual

Breakpoint Fault

Format

opcode

Condition Codes

Exceptions

• None.

Opcodes

Description

To understand the operation of this instruction, refer to Appendix E. This instruction, together with the T-bit, is used to implement debugging facilities.

Halt

Format

opcode

Condition Codes

Exceptions

• privileged instruction

Opcodes

Description

If the process is running in kernel mode, the processor is halted. Otherwise, a privileged instruction fault occurs. For information about privileged instruction faults, refer to Appendix E.

Note

This opcode is zero to trap many branches to data.

Compute Index

Format

opcode subscript.rl, low.rl, high.rl, size.rl, indexin.rl,

indexout.wl

Condition Codes

Exceptions

• subscript range

Opcodes

Description

The indexin operand is added to the subscript operand and the sum multiplied by the size operand. The indexout operand is replaced by the result. If the subscript operand is less than the low operand or greater than the high operand, a subscript range trap is taken.

Notes

1. No arithmetic exception other than subscript range can result from this instruction. Therefore, no indication is given if overflow occurs in either the add or the multiply steps. If overflow occurs on the add step, the sum is the low-order 32 bits of the true result. If overflow occurs on the multiply step, the indexout operand is replaced by the low-order 32 bits of the true product of the sum and the subscript operand. In the normal use of this instruction, overflow cannot occur without a subscript range trap occurring.
2. The index instruction is useful in index calculations for arrays of the fixed-length data types (integer and floating) and for index calculations for arrays of bit fields, character strings, and decimal strings. The indexin operand permits cascading INDEX instructions for multidimensional arrays. For one-dimensional bit field arrays, it also permits introduction of the constant portion of an index calculation that is not readily absorbed by address arithmetic. The following notes show some of the uses of INDEX.

3. The following example shows a sequence of COBOL statements and the VAX MACRO code their compilation might generate:

   COBOL: 01 A-ARRAY. 02 A PIC X(10) OCCURS 15 TIMES. 01 B PIC X(10). MOVE A(I) TO B.

   MACRO: INDEX I, #1, #15, #10, #0, R0 MOVC3 #10, A-10[R0], B.

4. The following example shows a sequence of PL/I statements and the VAX MACRO code their compilation might generate:

   PL/I: DCL A(-3:10) BIT (5); A(I) = 1;

   MACRO: INDEX I, #-3, #10, #5, #3, R0 INSV #1, R0, #5, A ; Assumes A is byte aligned

5. The following example shows a sequence of FORTRAN statements and the VAX MACRO code their compilation might generate:

   FORTRAN: INTEGER*4 A(L1:U1, L2:U2), I, J A(I,J) = 1

   MACRO: INDEX J, #L2, #U2, #M1, #0, R0; M1=U1-L1+1 INDEX I, #L1, #U1, #1, R0, R0; MOVL #1, A-a[R0]; a = {{L2*M1} + L1} *4

Move from PSL

Format

opcode dst.wl

Condition Codes

Exceptions

• None.

Opcodes

Description

The destination operand is replaced by processor status longword (PSL).

No Operation

Format

opcode

Condition Codes

Exceptions

• None.

Opcodes

Description

No operation is performed. Because the time delay caused by a NOP instruction is dependent on processor type, Digital recommends that you do not use NOP as a means of delaying program execution. When you must have a program wait for a specified period, you should use a macro, such as the TIMEDWAIT macro, or code sequence that is not dependent on the processor's internal speed.

Pop Registers

Format

opcode mask.rw

Condition Codes

Exceptions

• None.

Opcodes

Description

The contents of registers whose numbers correspond to set bits in the mask operand are replaced by longwords popped from the stack. R[n] is replaced if mask<n> is set. The mask is scanned from bit 0 to bit 14. Bit 15 is ignored.

Push Registers

Format

opcode mask.rw

Condition Codes

Exceptions

• None.

Opcodes

Description

The contents of registers whose numbers correspond to set bits in the mask operand are pushed on the stack as longwords. R[n] is pushed if mask<n> is set. The mask is scanned from bit 14 to bit 0. Bit 15 is ignored.

Note

The order of pushing is specified so that the contents of higher-numbered registers are stored at higher memory addresses. An example of a result of this would be a double-floating datum stored in adjacent registers being stored by PUSHR in memory in the correct order.

Extended Function Call

Format

opcode

Condition Codes

Exceptions

• None.

Opcodes

Description

To understand the operation of this instruction, refer to Appendix E and the VAX Architecture Reference Manual. This instruction provides for customer-defined extensions to the instruction set.

9.2.7 Queue Instructions

A queue is classified by the type of link that it uses. The VAX supports two distinct types of links: absolute and self-relative.

9.2.7.1 Absolute Queues

Absolute queues use absolute addresses as links. Queue entries are linked by a pair of longwords. The first (lowest-addressed) longword is the forward link; it is the address of the succeeding queue entry. The second (highest-addressed) longword is the backward link; it is the address of the preceding queue entry.

A queue is specified by a queue header, which is identical to a pair of queue linkage longwords. The forward link of the header is the address of the entry called the head of the queue. The backward link of the header is the address of the entry termed the tail of the queue. The forward link of the tail points to the header.

Two general operations can be performed on queues: insertion of entries and removal of entries. Generally, entries can be inserted or removed only at the head or tail of a queue. (Under certain restrictions they can be inserted or removed elsewhere; this is discussed later.)

The following text contains examples of queue operations. An empty queue is specified by its header at address H.

If an entry at address B is inserted into an empty queue (at either the head or the tail), the queue appears as follows:

If an entry at address A is inserted at the head of the queue, the queue appears as follows:

Finally, if an entry at address C is inserted at the tail, the queue appears as follows:

Following the preceding steps in reverse order gives the effect of removal at the tail and removal at the head.

If more than one process can perform operations on a queue simultaneously, insertions and removals should only be done at the head or tail of the queue. If only one process (or one process at a time) can perform operations on a queue, insertions and removals can be made at other than the head or tail of the queue. In the preceding example with the queue containing entries A, B, and C, the entry at address B can be removed, giving the following:

The reason for this restriction is that operations at the head or tail are always valid because the queue header is always present. Operations elsewhere in the queue depend on specific entries being present and may become invalid if another process is simultaneously performing operations on the queue.

Two instructions are provided for manipulating absolute queues: INSQUE and REMQUE. INSQUE inserts an entry specified by an entry operand into the queue following the entry specified by the predecessor operand. REMQUE removes the entry specified by the entry operand. Queue entries can be on arbitrary byte boundaries. Both INSQUE and REMQUE are implemented as noninterruptible instructions.

9.2.7.2 Self-Relative Queues

Self-relative queues use displacements from queue entries as links. Queue entries are linked by a pair of longwords. The first (lowest addressed) longword is the forward link; it is the displacement of the succeeding queue entry from the present entry. The second (highest-addressed) longword is the backward link; it is the displacement of the preceding queue entry from the present entry.

A queue is specified by a queue header, which also consists of two longword links. The forward link of the header is the address of the entry called the head of the queue. The backward link of the header is the address of the entry called the tail of the queue. The forward link of the tail points to the header.

The following text contains examples of queue operations. An empty queue is specified by its header at address H. Because the queue is empty, the self-relative links must be zero, as shown.

If an entry at address B is inserted into an empty queue (at either the head or tail), the queue appears as follows:

If an entry at address A is inserted at the head of the queue, the queue appears as follows:

Finally, if an entry at address C is inserted at the tail, the queue appears as follows:

Following the previous steps in reverse order gives the effect of removal at the tail and at the head.

The following four instructions manipulate self-relative queues:

1. INSQHI---Insert entry into queue at head, interlocked.
2. INSQTI---Insert entry into queue at tail, interlocked.
3. REMQHI---Remove entry from queue at head, interlocked.
4. REMQTI---Remove entry from queue at tail, interlocked.

These operations are interlocked to allow cooperating processes in a multiprocessor system to access a shared list without additional synchronization. Queue entries must be quadword aligned. A hardware-supported interlocked memory access mechanism is used to read the queue header. Bit 0 of the queue header is used as a secondary interlock; it is set when the queue is being accessed.

If an interlocked queue instruction encounters the secondary interlock set, then, if no exception conditions exist, it terminates after setting the condition codes to indicate failure to gain access to the queue. If the secondary interlock bit is not set, then the interlocked queue instruction sets the secondary interlock bit during instruction execution and clears the secondary interlock bit at instruction completion. In this way, other interlocked queue instructions are prevented from operating on the same queue.

If an interlocked queue instruction encounters both the secondary interlock set and an exception condition resulting from instruction execution, then it is UNPREDICTABLE whether the exception occurs or the instruction terminates after setting the condition codes.

9.2.7.3 Instruction Descriptions

Insert Entry into Queue at Head, Interlocked

Format

opcode entry.ab, header.aq

Condition Codes

Exceptions

• reserved operand

Opcodes

Description

The entry specified by the entry operand is inserted into the queue following the header. If the entry inserted was the first one in the queue, the condition code Z-bit is set; otherwise it is cleared. The insertion is a noninterruptible operation. The insertion is interlocked to prevent concurrent interlocked insertions or removals at the head or tail of the same queue by another process even in a multiprocessor environment. Before performing any part of the operation, the processor validates that the entire operation can be completed. This method ensures that if a memory management exception occurs (see Appendix E), the queue is left in a consistent state. If the instruction fails to acquire the secondary interlock, then, if no exception conditions exist, the instruction sets condition codes and terminates.

Notes

1. Because the insertion is noninterruptible, processes running in kernel mode can share queues with interrupt service routines.
2. The INSQHI, INSQTI, REMQHI, and REMQTI instructions are implemented such that cooperating software processes in a multiprocessor may access a shared list without additional synchronization.
3. To set a software interlock realized with a queue, you can use the following:

   INSERT: INSQHI ... ; Was queue empty? BEQL 1$ ; Yes BCS INSERT ; Try inserting again CALL WAIT(...) ; No, wait 1$:

4. During access validation, any access that cannot be completed results in a memory management exception even though the queue insertion is not started.
5. A reserved operand fault occurs if entry or header is an address that is not quadword aligned (that is, <2:0> NEQU 0) or if header<2:1> is not zero. A reserved operand fault also occurs if header equals entry. In this case, the queue is not altered.
6. If an interlocked queue instruction encounters both the secondary interlock set and an exception condition resulting from instruction execution, then it is UNPREDICTABLE whether the exception occurs or the instruction terminates after setting the condition codes.

Copyright © Compaq Computer Corporation 1998. All rights reserved. Legal

4515PRO_021.HTML