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

TNS Execution Modes

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

6-28

SG Addressing Mode

For memory-reference instructions, an additional addressing mode is provided, called

the SG-relative mode. This mode is provided to permit access, from the user

environment, to system global data in the system data segment. This mode directly

addresses the first 64 locations of the system data segment (SG[0:63]), and indirectly

can address the entire 64K-word segment. Refer to Figure 6-17.

The SG-relative mode is indicated when bits 7 through 9 of a memory reference

instruction are equal to 110 (binary). Bits 10 through 15 are a positive word

displacement from SG[0]. The specification of SG-relative addressing by the

instruction overrides the DS bit setting in the Environment register; that is, the system

data segment will be accessed even if the DS bit indicates user data.

Use of the SG-relative addressing mode is restricted to privileged mode only, and thus

can be used only by callable and privileged procedures. Such procedures, when they

operate on the user’s data stack, are able to load, store, move, compare, and scan

data, using both the user data segment and the system data segment.

Indirect addressing and indexing are both permitted with the SG-relative addressing

mode.

SG-relative addressing mode is specified by 110 in the mode field of the memory

reference instruction. The first example shows a direct reference to SG[4], as

indicated by the value in the displacement field. The second example shows an

indirect reference to SG[42176], through the direct location SG[13]. (Addresses are in

decimal.) The X field specifies an index register, if used.