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2.5 Operand Selection

An instruction can act on zero or more operands, which are the data manipulated by the instruction. An example of a zero-operand instruction is NOP (no operation). An operand can be in any of these locations: Immediate operands and operands in registers can be accessed more rapidly than operands in memory since memory operands must be fetched from memory. Register operands are available in the CPU. Immediate operands are also available in the CPU, because they are prefetched as part of the instruction.

Of the instructions that have operands, some specify operands implicitly; others specify operands explicitly; still others use a combination of implicit and explicit specification; for example:

Implicit operand: AAM
By definition, AAM (ASCII adjust for multiplication) operates on the contents of the AX register.
Explicit operand: XCHG EAX, EBX
The operands to be exchanged are encoded in the instruction after the opcode.
Implicit and explicit operands: PUSH COUNTER
The memory variable COUNTER (the explicit operand) is copied to the top of the stack (the implicit operand).
Note that most instructions have implicit operands. All arithmetic instructions, for example, update the EFLAGS register.

An 80386 instruction can explicitly reference one or two operands. Two-operand instructions, such as MOV, ADD, XOR, etc., generally overwrite one of the two participating operands with the result. A distinction can thus be made between the source operand (the one unaffected by the operation) and the destination operand (the one overwritten by the result).

For most instructions, one of the two explicitly specified -- the source or the -- be either in a register or in memory. The other operand must be in a register or be an immediate source operand. Thus, the explicit two-operand instructions of the 80386 permit operations of the following kinds:

Certain string instructions and stack manipulation instructions, however, transfer data from memory to memory. Both operands of some string instructions are in memory and are implicitly specified. Push and pop stack operations allow transfer between memory operands and the memory-based stack.

2.5.1 Immediate Operands

Certain instructions use data from the instruction itself as one (and sometimes two) of the operands. Such an operand is called an immediate operand. The operand may be 32-, 16-, or 8-bits long. For example: 
SHR PATTERN, 2
One byte of the instruction holds the value 2, the number of bits by which to shift the variable PATTERN. 
TEST PATTERN, 0FFFF00FFH
A doubleword of the instruction holds the mask that is used to test the variable PATTERN.

2.5.2 Register Operands

Operands may be located in one of the 32-bit general registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, or EBP), in one of the 16-bit general registers (AX, BX, CX, DX, SI, DI, SP, or BP), or in one of the 8-bit general registers (AH, BH, CH, DH, AL, BL, CL,or DL).

The 80386 has instructions for referencing the segment registers (CS, DS, ES, SS, FS, GS). These instructions are used by applications programs only if systems designers have chosen a segmented memory model.

The 80386 also has instructions for referring to the flag register. The flags may be stored on the stack and restored from the stack. Certain instructions change the commonly modified flags directly in the EFLAGS register. Other flags that are seldom modified can be modified indirectly via the flags image in the stack.

2.5.3 Memory Operands

Data-manipulation instructions that address operands in memory must specify (either directly or indirectly) the segment that contains the operand and the offset of the operand within the segment. However, for speed and compact instruction encoding, segment selectors are stored in the high speed segment registers. Therefore, data-manipulation instructions need to specify only the desired segment register and an offset in order to address a memory operand.

An 80386 data-manipulation instruction that accesses memory uses one of the following methods for specifying the offset of a memory operand within its segment:

  1. Most data-manipulation instructions that access memory contain a byte that explicitly specifies the addressing method for the operand. A byte, known as the modR/M byte, follows the opcode and specifies whether the operand is in a register or in memory. If the operand is in memory, the address is computed from a segment register and any of the following values: a base register, an index register, a scaling factor, a displacement. When an index register is used, the modR/M byte is also followed by another byte that identifies the index register and scaling factor. This addressing method is the most flexible.
  2. A few data-manipulation instructions implicitly use specialized addressing methods:

2.5.3.1 Segment Selection

Data-manipulation instructions need not explicitly specify which segment register is used. For all of these instructions, specification of a segment register is optional. For all memory accesses, if a segment is not explicitly specified by the instruction, the processor automatically chooses a segment register according to the rules of Table 2-1. (If systems designers have chosen a flat model of memory organization, the segment registers and the rules that the processor uses in choosing them are not apparent to applications programs.)

There is a close connection between the kind of memory reference and the segment in which that operand resides. As a rule, a memory reference implies the current data segment (i.e., the implicit segment selector is in DS). However, ESP and EBP are used to access items on the stack; therefore, when the ESP or EBP register is used as a base register, the current stack segment is implied (i.e., SS contains the selector).

Special instruction prefix elements may be used to override the default segment selection. Segment-override prefixes allow an explicit segment selection. The 80386 has a segment-override prefix for each of the segment registers. Only in the following special cases is there an implied segment selection that a segment prefix cannot override: 

Table 2-1. Default Segment Register Selection Rules

Memory Reference Needed  Segment     Implicit Segment Selection Rule
Register
Used

Instructions             Code (CS)   Automatic with instruction prefetch
Stack                    Stack (SS)  All stack pushes and pops. Any
memory reference that uses ESP or
EBP as a base register.
Local Data               Data (DS)   All data references except when
relative to stack or string
destination.
Destination Strings      Extra (ES)  Destination of string instructions.

2.5.3.2 Effective-Address Computation

The modR/M byte provides the most flexible of the addressing methods, and instructions that require a modR/M byte as the second byte of the instruction are the most common in the 80386 instruction set. For memory operands defined by modR/M, the offset within the desired segment is calculated by taking the sum of up to three components:

The offset that results from adding these components is called an effective address. Each of these components of an effective address may have either a positive or negative value. If the sum of all the components exceeds 2^(32), the effective address is truncated to 32 bits. Figure 2-10 illustrates the full set of possibilities for modR/M addressing.

The displacement component, because it is encoded in the instruction, is useful for fixed aspects of addressing; for example:

The base and index components have similar functions. Both utilize the same set of general registers. Both can be used for aspects of addressing that are determined dynamically; for example: The uses of general registers as base or index components differ in the following respects: The scaling factor permits efficient indexing into an array in the common cases when array elements are 2, 4, or 8 bytes wide. The shifting of the index register is done by the processor at the time the address is evaluated with no performance loss. This eliminates the need for a separate shift or multiply instruction.

The base, index, and displacement components may be used in any combination; any of these components may be null. A scale factor can be used only when an index is also used. Each possible combination is useful for data structures commonly used by programmers in high-level languages and assembly languages. Following are possible uses for some of the various combinations of address components.

DISPLACEMENT
The displacement alone indicates the offset of the operand. This combination is used to directly address a statically allocated scalar operand. An 8-bit, 16-bit, or 32-bit displacement can be used.
BASE
The offset of the operand is specified indirectly in one of the general registers, as for "based" variables.
BASE + DISPLACEMENT
A register and a displacement can be used together for two distinct purposes:
  1. Index into static array when element size is not 2, 4, or 8 bytes. The displacement component encodes the offset of the beginning of the array. The register holds the results of a calculation to determine the offset of a specific element within the array.
  2. Access item of a record. The displacement component locates an within record. The base register selects one of several occurrences of record, thereby providing a compact encoding for this common function.
An important special case of this combination, is to access parameters in the procedure activation record in the stack. In this case, EBP is the best choice for the base register, because when EBP is used as a base register, the processor automatically uses the stack segment register (SS) to locate the operand, thereby providing a compact encoding for this common function.
(INDEX * SCALE) + DISPLACEMENT
This combination provides efficient indexing into a static array when the element size is 2, 4, or 8 bytes. The displacement addresses the beginning of the array, the index register holds the subscript of the desired array element, and the processor automatically converts the subscript into an index by applying the scaling factor.
BASE + INDEX + DISPLACEMENT
Two registers used together support either a two-dimensional array (the displacement determining the beginning of the array) or one of several instances of an array of records (the displacement indicating an item in the record).
BASE + (INDEX * SCALE) + DISPLACEMENT
This combination provides efficient indexing of a two-dimensional array when the elements of the array are 2, 4, or 8 bytes wide.

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