TAL Programmer's Guide

Addressing Modes

Introducing the Environment

096254 Tandem Computers Incorporated 4–5

System Code Space The current system code space available to your process contains one segment of

system code.

System Library Space The current system library space contains up to 31 segments of system library code,

available to your process one at a time.

Registers Table 4-1 lists the registers that describe your process in the current process

environment:

Table 4-1. Registers in Current Process Environment

Program (P) register Contains the address of the next instruction to execute in the current

code segment

Instruction (I) register Contains the instruction currently executing in the current code segment

Local (L) register Contains the address of the beginning of the local data area for the

current procedure

Stack (S) register Contains the address of the last allocated word in the data stack for the

current procedure or subprocedure

and other operations. The compiler also uses R5, R6, and R7 as index

registers.

Environment (E) register Contains information about the current process, such as the current RP

value and whether traps are enabled

Addressing Modes When you declare a variable, you specify its addressing mode and an identifier and a

data type. You can specify these addressing modes:

Direct addressing

Standard indirect addressing

Extended indirect addressing

The addressing mode and scope of a variable determines:

The storage area in which the compiler allocates space for the variable

The kind of instructions the compiler generates to access the variable

The data type of a variable determines the byte or word addressing mode of the

variable. Figure 4-3 shows byte and word addresses in the user data segment.

(Extended segments are always byte addressed.)