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 language chooses a short-indexed reference. If the value is greater than eight bits, it chooses a longindexed 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 establish 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 compatibility 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. Memorymapped 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 “Specialfunction Registers (SFRs)” on page 4-5 and “Register File” on page 4-10 for more information.

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8XC196Kx, Jx, CA USER’S MANUAL

To use these registers effectively, you must have some overall strategy for allocating them. The C programming language adopts a simple, effective strategy. It allocates the eight bytes beginning at address 1CH as temporary storage and treats the remaining area in the register file as a segment of memory that is allocated as required.

NOTE

Using any SFR as a base or index register for indirect or indexed operations can cause unpredictable results. External events can change the contents of SFRs, and some SFRs are cleared when read. For this reason, consider the implications of using an SFR as an operand in a read-modify-write instruction (e.g., XORB).

3.4.2Addressing 32-bit Operands

The 32-bit operands (DOUBLE-WORDs and LONG-INTEGERs) are formed by two adjacent 16-bit words in memory. The least-significant word of a DOUBLE-WORD is always in the lower address, even when the data is in the stack (which means that the most-significant word must be pushed into the stack first). The address of a 32-bit operand is that of its least-significant byte.

The hardware supports the 32-bit data types as operands in shift operations, as dividends of 32- by-16 divide operations, and as products of 16-by-16 multiply operations. For these operations, the 32-bit operand must reside in the lower register file and must be aligned at an address that is evenly divisible by four.

3.4.3Linking Subroutines

Parameters are passed to subroutines via the stack. Parameters are pushed into the stack from the rightmost parameter to the left. The 8-bit parameters are pushed into the stack with the high-order byte undefined. The 32-bit parameters are pushed onto the stack as two 16-bit values; the mostsignificant half of the parameter is pushed into the stack first. As an example, consider the following procedure:

void example_procedure (char param1, long param2, int param3);

When this procedure is entered at run-time, the stack will contain the parameters in the following order:

param3

low word of param2

high word of param2

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