Intel 210200-002 manual Addressing Modes

ARCHITECTURE AND INSTRUCTIONS

3)Setting TF (the trap flag) puts the proces- sor into single-step mode for debugging. In this mode, the CPU automatically generates an internal interrupt after each instruction, allowing a program to be inspected as it exe- cutes instruction by instruction.

Instruction Pointer

The 16-bit instruction pointer (IP), as shown in Figure 2-2, is analogous to the program counter (PC) in the 8080/8085 CPUs and points to the next instruction. The instruction pointer contains the offset (distance in bytes) of the next instruction from the beginning of the current code segment. During normal execution, IP contains the offset of the next instruction to be fetched. Whenever IP is saved on the stack, however, it first is auto- matically adjusted to point to the next instruction to be executed. Programs do not have direct access to the instruction pointer, but instructions cause it to change and to be saved on and restored from the stack.

Stack Implementation

The 8088's stack is implemented in memory and is located by the stack segment register

(SS)and the stack pointer register (SP). A system may have an unlimited number of stacks, and a stack may be up to 64K bytes long, the maximum length of a segment. (An attempt to expand a stack beyond 64K bytes overwrites the beginning of the stack). One stack is directly addressable at a time; this is the current stack often referred to simply as "the" stack. SS contains the base address of the current stack and SP points to the top of the stack (TOS). In other words, SP contains the offset of the top of the stack from the stack segment's base address. Note, however, that the stack's base address (contained in SS) is not the "bottom" of the stack.

Instructions that operate on a stack add or remove one word (2 bytes) at a time. An item is pushed onto the stack by decrementing SP by 2 and writing the item at the new TOS. An

item is popped off the stack by copying it from TOS and incrementing SP by 2. In other words, the stack grows' down in memory toward its base address: Stack oper- ations never move items on the stack, nor do they t:rase them. The top of tbe stack changes only as a result of updating the stack pointer.

ADDRESSING MODES

Instructions in the 8088 usually perform operations on one or two source operands, with the result overwriting one of the oper- ands. The first operand of a two-operand instruction can be usually either a register or a memory location; the second operand can be either a register or a constant within the instruction. (The terms first and second oper- and are used to distinguish the operands only

-their use does not imply directionality for data transfers). Typical formats for two- operand instructions are shown in Figure 2-4.

Single-operand instructions generally allow either a register or a memory location to serve as the operand. Figure 2-4 also shows a typical one-operand format. Virtually all 8088 operators may, specify 8- or 16-bit operands.

Memory Operands,

An instruction may address an operand resid- ing in memory in one of the following ways, as determined by the "mod" and "r/ m" field in the instruction (Fig. 2-5):

DIRECT ADDRESSING -16-bit offset address contained in the instruction.

INDIRECT ADDRESSI~G - optionally with an 8- or 16-bit displacement contained in the instruction:

1)through a base register (BP or BX)

2)through an index register (SI or DI)

3)through the sum of a base register and an index register

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