## ALP Programming Guide and Reference

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### Indexed Expression Type

An Indexed-ExpressionType is an expression that calculates an effective memory address using the contents of a Base-Register, an Index-Register, or both. A Processor-Register must first be converted to a Base-Register or Index-Register by specifying it as the operand of the Indirection ([] Operator) before the expression can be converted to an Indexed- ExpressionType.

When calculating a 16-bit effective address, only the BP and BX registers may be used as Base-Registers, and only the DI and SI registers may be used as Index-Registers.

When calculating a 32-bit effective address, only the EAX, EBX, ECX, EDX, EDI, ESI, EBP, and ESP registers may be used as Base-Registers, and only the EAX, EBX, ECX, EDX, EDI, ESI, and EBP registers may be used as Index- Registers.

Note: Only a single Base-Register and a single Index-Register may be used in a given expression.

On 80386 (and higher) processors, the Multiplication (* Operator) may be used with an Index-Register operand and an Absolute-ExpressionType operand to establish a scaling factor that is applied to the Index-Register during effective address calculation. The scaling factor effectively causes the Index-Register to be multiplied by a fixed value at run time. The scaling Expression must evaluate to 1 (no scale factor), 2, 4, or 8.

A Direct-ExpressionType or an Indirect-ExpressionType may be a sub- expression of an Indexed-ExpressionType.