User's Manual

THE 80286 INSTRUCTION SET
ENTER-Make Stack Frame for Procedure Parameters
Opcode
C8 dw 00
C8 dw 01
C8 dw db
Instruction
ENTER dw,O
ENTER dW,1
ENTER dW,db
FLAGS MODIFIED Cc
None
FLAGS UNDEFINED
None
OPERATION
Clocks
11
15
12+4db
Description
Make stack frame for procedure parameters
Make stack frame
for procedure parameters
Make stack frame for procedure parameters
ENTER is used to create the stack frame required by most block-structured high-level languages. The
first operand specifies
how many bytes of dynamic storage are to be allocated on the stack for the
routine being entered. The second operand gives the lexical nesting level of the routine within the high-
level-language source code. It determines how many stack frame pointers are copied into the new stack
frame from the preceding frame.
BP is used as the current stack frame pointer.
If the second operand is 0, ENTER pushes BP, sets BP to SP, and subtracts the first operand from
SP.
For example, a procedure with 12 bytes of local variables would have an ENTER 12,0 instruction at
its entry point and a LEAVE instruction before every RET. The 12 local byteswou!d be addressed as
negative offsets from
[BPj. See also section 4.2.
The formal definition of the
ENTER instruction for all cases is given by the following listing. LEVEL
denotes the value of the second operand.
LEVEL: = LEVEL MOD 32
Push BP
Set a temporary value FRAM~PTR : = sp
If LEVEL> 0 then
Repeat
(LEVEL -1) times:
BP:= BP - 2
. Push the word pOinted to by BP
End repeat
Push
FRAM~PTR
End if
BP:= FRAME_PTR
SP : = SP - first operand.
B-40