Intel 80286, 80287 manual Inter

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80286 BASE ARCHITECTURE

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LOGICAL

BOTTOM OF STACK (initial SP value)

POP-UP

LOGICAL

TOP OF STACK

PUSH-DOWN

I SS I SP I

STACK SEGMENT BASE ADDRESS

G3010B

Figure 2-7_80286 Stack

80286 stack entries are 16 bits wide. Instructions operate on the stack by adding and removing stack items one word at a time, An item is pushed onto the stack (see figure 2-8) 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 then incrementing SP by 2. In other words, the stack grows down in memory toward its base address. Stack operations never move items on the stack; nor do they erase them. The top of the stack changes only as a result of updating the stack pointer.

The stack frame base pointer (BP) is often used to access elements on the stack relative to a fixed point on the stack rather than relative to the current TOS. It typically identifies the base address of the current stack frame established for the current procedure (figure 2-9). If an index register is used relative to BP (e.g., base + index addressing mode using BP as the base), the offset will be calculated automatically in the current stack segment.

Accessing data structures in data segments is facilitated by the BX register, which has the same function in addressing operands within data segments that BP does for stack segments. They are called base registers because they may contain an offset to the base of a data structure. The similar usage of these two registers is especially important when discussing addressing modes (see section 2.4, "Addressing Modes").

Operations on data are also facilitated by the SI and DI registers. By specifying an offset relative to the start of the currently addressable data segment, an index register can be used to address an operand in the segment. If an index register is used in conjunction with the BX base register (i.e., base + index addressing) to form an offset address, the data is also assumed to reside in the current data segment. As a rule, data referenced through an index register or BX is presumed to reside in the current data segment. That is, if an instruction invokes addressing for one of its operands using either BX, DI, SI, or BX with SI or DI, the contents of the register(s) (BX, DI, or SI) implicitly specify an offset in the current data segment. As previously mentioned, data referenced via SP, BP or BP with SI or DI implic- itly specify an operand in the current stack segment (refer to table 2-1).

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