NonStop S-Series Server Description Manual (G06.27+)

TNS Execution Modes

HP NonStop S-Series Server Description Manual—520331-004

6-16

Branching, Direct and Indirect

Addresses for branching (and for constants) in a code segment are calculated relative

to the current setting of the P register. This is referred to as P-relative addressing.

Instructions that reference a code segment have an eight-bit field (seven magnitude

bits plus a sign) for specifying a relative displacement from the current P register

setting. The range of the displacement is therefore –128:+127 words. An example,

the BUN instruction, is shown in Figure 6-9.

A branch can be either direct or indirect (an example of each is shown in Figure 6-10).

In computing a branch target address, compilers first add the displacement to the

current P register setting. This value is the direct branch address. If the referenced

location is within the range of the displacement (that is, within the range P

[-128:+127]), then direct addressing is indicated and the direct branch address is

used as the branch address. Direct addressing is specified when the indirect bit (bit

0) of the instruction is equal to 0.

If the referenced location is beyond the range of the displacement, the compiler

generates an indirect branch and the referenced location is a relative displacement

from the direct branch address. That is, the contents of the word at the direct branch

address (containing a displacement from itself) are added to the direct branch address.

The result is the target C[0]-relative branch address. Because a 16-bit word is

available to specify a displacement (not just seven bits, as in direct addressing), the

jump range is extended to the entire TNS code segment. Indirect addressing is

specified when the indirect bit of the instruction is equal to 1.

Figure 6-9. The BUN Instruction Is Typical of Branch Instructions

BUN (Branch Unconditionally) Instruction Format:

23456789101112131415

Displacement

0010001

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