NonStop S-Series Server Description Manual (G06.24+)

TNS Execution Modes

HP NonStop S-Series Server Description Manual—520331-003

6-24

Indexing in the Data Segment

Indexing is used to reference memory locations relative to a data element in memory.

A typical use is the accessing of an element in an array.

Generally, indexing is done as follows. An initial address is first calculated as

described in the preceding two topics (any addressing mode as well as direct, indirect,

and byte addressing is permitted). This initial address is then used as a base address

for indexing. The indexing value, contained in an index register (referred to as “X”), is

added to the initial address to provide the address of the referenced operand. This is

shown in the upper part of Figure 6-15.

Any one of three registers in the register stack (R[5:7]) can be used as an index

(bits 5 and 6) in all memory reference instructions. (Note the instruction format in the

lower part of the figure.) The index register field corresponds to register stack

elements as follows:

An index register can contain values from –32,768 through +32,767 to provide direct

word and doubleword addressing of any location in the data area (all addressing is

modulo 65,536). Byte addressing, as described in the preceding topic, applies only in

the first half of the data segment.

The value in an index register is always treated as an element indexing value. That is,

if a byte instruction is being executed, the content of an index register is treated as a

byte offset; if a doubleword instruction is being executed, the content is treated as a

doubleword offset. The next topic shows examples of word and byte indexing.

Index Register Field Index Register

0 X = no indexing

1X = R[5]

2X = R[6]

3X = R[7]