User's Manual

BASIC INSTRUCTION seT
3.1.2 Stack Manipulation Instructions
PUSH (Push) decrements the stack pointer (SP) by two and then transfers a word from the source
operand to the top of stack indicated by
SP. See figure 3-1. PUSH is often used to place parameters
on the stack before calling a procedure; it
is also the basic means of storing temporary variables on the
stack. The
PUSH instruction operates on memory operands, immediate operands (new with the 80286),
and register operands (including segment registers).
Example:
PUSH WORDOPRND. Transfers a 16-bit value from the memory word identified by the
label WORDOPRND to the memory location which represents the current top of stack
(byte transfers are not allowed).
PUSHA (Push All Registers) saves the contents of the eight general registers on the stack. See
figure 3-2. This instruction simplifies procedure calls by reducing the number of instructions required
to retain the contents of the general registers for use in a procedure.
PUSHA is complemented by
POPA (see below).
The processor pushes the general registers
on the stack in the following order: AX, ex, DX, BX, the
initial value of
SP before AX was pushed, BP, sr, and Dr.
Example:
PUSHA. Pushes onto the stack the contents of the eight general registers.
HIGH ADDRESS
OPERANDS FROM
PREVIOUS PUSH
INSTRUCTIONS
LOW ADDRESS
OPERAND
SS LIMIT
SP ALWAYS POINTS TO
_ THE LAST WORD PUSHED
ONTO THE STACK (TOS)
SS ALWAYS POINTS TO
LOWEST ADDRESS USED BY
THE STACK
1 i! i
BEFORE
AFTER
PUSH OPERAND
PUSH OPERAND
PUSH decrements SP by 2 bytes and places the operand In the stack at the location to which SP points.
G3010B
Figure 3-1. PUSH
3-2