Intel 8XC196MD Assembly Language Addressing Mode Selections, Software Standards and Conventions

PROGRAMMING CONSIDERATIONS

3.3ASSEMBLY LANGUAGE ADDRESSING MODE SELECTIONS

The assembly language simplifies the choice of addressing modes. Use these features wherever possible.

3.3.1Direct Addressing

The assembly language chooses between direct and zero-indexed addressing depending on the memory location of the operand. Simply refer to the operand by its symbolic name. If the operand is in the lower register file, the assembly language chooses a direct reference. If the operand is elsewhere in memory, it chooses a zero-indexed reference.

3.3.2Indexed Addressing

The assembly language chooses between short-indexed and long-indexed addressing depending on the value of the index expression. If the value can be expressed in eight bits, the assembly lan- guage chooses a short-indexed reference. If the value is greater than eight bits, it chooses a long- indexed reference.

3.4SOFTWARE STANDARDS AND CONVENTIONS

For a software project of any size, it is a good idea to develop the program in modules and to es- tablish standards that control communication between the modules. These standards vary with the needs of the final application. However, all standards must include some mechanism for passing parameters to procedures and returning results from procedures. We recommend that you use the conventions adopted by the C programming language for procedure linkage. These standards are usable for both the assembly language and C programming environments, and they offer compat- ibility between these environments.

3.4.1Using Registers

The 256-byte lower register file contains the CPU special-function registers and the stack pointer. The remainder of the lower register file and all of the upper register file is available for your use. Peripheral special-function registers (SFRs) and memory-mapped SFRs reside in higher memory. The peripheral SFRs can be windowed into the lower register file for direct access. Memory- mapped SFRs cannot be windowed; you must use indirect or indexed addressing to access them. All SFRs can be operated on as BYTEs or WORDs, unless otherwise specified. See “Special- function Registers (SFRs)” on page 4-4 and “Register File” on page 4-9 for more information.

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