Intel 80287, 80286 manual HIGH-LEVEL Instructions

EXTENDED INSTRUCTION SET

OUTS (Output String to Port) transfers a byte or a word string element to an output port from memory. Combined with the REP prefix, OUTS moves a block of information from a series of consecutive memory locations indicated by DS:SI to an output port.

Example: REP OUTS WSTRING. Assuming that the program declares WSTRING to be a word- length string element, the assembler uses the 16-bit form of the OUTS instruction to create the object code for the program. The processor repeatedly transfers words from the memory locations indicated by DI to the output port selected by the 16-bit port number in DX.

Following each word transfer, the CPU decrements CX. The instruction terminates the block transfer when CX=O. After decrementing CX, the processor increments SI by two to point to the next word in memory if DF=O; it decrements SI by two if DF= 1.

4.2 HIGH-LEVEL INSTRUCTIONS

The instructions in this section provide machine-language functions normally found only in high-level languages. These instructions include ENTER and LEAVE, which simplify the programming of proce- dures, and BOUND, which provides a simple method of testing an index against its predefined range.

ENTER (Enter Procedure) creates the stack frame required by most block-structured high-level languages. A LEAVE instruction at the end of a procedure complements an ENTER at the beginning of the procedure to simplify stack management and to control access to variables for nested procedures.

Example: ENTER 2048,3. Allocates 2048 bytes of dynamic storage on the stack and sets up pointers to two previous stack frames in the stack frame that ENTER creates for this procedure.

The ENTER instruction includes two parameters. The first parameter specifies the number of bytes of dynamic storage to be allocated on the stack for the routine being entered. The second parameter corresponds to the lexical nesting level (0-31) of the routine. (Note that the lexical level has no relation- ship to either the protection privilege levels or to the I/O privilege level.)

The specified lexical level determines hovv' many sets of stack frame pointers the CPU copies into the

new stack frame from the preceding frame. This list of stack frame pointers is sometimes called the "display." The first word of the display is a pointer to the last stack frame. This pointer enables a LEAVE instruction to reverse the action of the previous ENTER instruction by effectively discarding the last stack frame.

After ENTER creates the new display for a procedure, it allocates the dynamic storage space for that procedure by decrementing SP by the number of bytes specified in the first parameter. This new value of:SP serves as a base for aU PUSH and POP operaiions wiihin ihai pro\;!:uun:.

To enable a procedure to address its display, ENTER leaves BP pointing to the beginning of the new stack frame. Data manipUlation instructions that specify BP as a base register implicitly address locations within the stack segment instead of tpe data segment. Two forms of the ENTER instruction exist: nested and non-nested. If the lexical level is 0, the non-nested form is used. Since the second operand is 0, ENTER pushes BP, copies SP to BP and then subtracts the first operand from SP. The nested form of ENTER occurs when the second parameter (lexical level) is not o. Figure 4-1 gives the formal definition of ENTER.

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