The Intel 80386, part 10: Atomic operations and memory alignment

Raymond ChenJanuary 31, 2019Jan 31, 201901/31/19

Memory access on the 80386 to misaligned locations are supported, although they will operate more slowly than their aligned counterparts. If a memory access straddles a page boundary, an access violation will be raised if either page does not support the desired operation (not readable or not writable), and the instruction will not appear to h

The Intel 80386, part 9: Stack frame instructions

Raymond ChenJanuary 30, 2019Jan 30, 201901/30/19

There are a pair of specialized instructions for creating and tearing down stack frames. The ENTER instruction sets up a stack frame for a new subroutine. It combines three instructions into one, so that what used to be encoded in eight bytes (1 + 2 + 5) is now encoded in four. However, even on the 80386, the combination instruction executes

The Intel 80386, part 8: Block operations

Raymond ChenJanuary 29, 2019Jan 29, 201901/29/19

Most of the special-purpose operations that the 80386 inherited from the 8086 are largely obsolete. Although processors still support them, the implementations are not optimized, and compilers don't generate them. Except for the block operations. Those are still important. The block operations (formally known as "string" instructions) operat

The Intel 80386, part 7: Conditional instructions and control transfer

Raymond ChenJanuary 28, 2019Jan 28, 201901/28/19

Finally we get to use the conditions that we defined way back in Part 2. The SETcc family of instructions sets an 8-bit value to 0 or 1 based on a condition code. For example, SETE al sets the al register to 1 if the ZF flag is set (equal), or to 0 if it is clear. If the destination of the SETcc instruction is an 8-bit register, it is typica

The Intel 80386, part 6: Data transfer instructions

Raymond ChenJanuary 27, 2019Jan 27, 201901/27/19

The most common data transfer operation is the simple copy of a value from one place to another. The MOV instruction does not support an 8-bit immediate with sign extension. The immediate must be the same size as the other operand. This is annoying if you want to move a small constant into a 32-bit register, because you have to burn four byte

The Intel 80386, part 5: Logical operations

Raymond ChenJanuary 24, 2019Jan 24, 201901/24/19

The next group of instructions we'll look are the bitwise logical operation. The AND, OR, and XOR instructions set flags based on the numeric value of the result; carry and overflow are always clear. The TEST instruction is the same as AND, except that the result is thrown away rather than being stored back into the destination. You can say

The Intel 80386, part 4: Arithmetic

Raymond ChenJanuary 23, 2019Jan 23, 201901/23/19

Okay, we've laid enough groundwork that we can start looking at instructions. Whereas arithmetic operations on most modern processors are three-operand (two sources and a destination), on the 80386, the arithmetic operations have only a single source and a single destination. The operations are performed in place, with the destination providi

The Intel 80386, part 3: Flags and condition codes

Raymond ChenJanuary 22, 2019Jan 22, 201901/22/19

The flags register contains a bunch of stuff, but here are the flags easily accessible in the debugger: We'll learn about the direction flag when we get to string operations. The important detail for now is that the direction flag must be clear (up) at function boundaries. Instructions for manipulating the interrupt flag are privileged, so

The Intel 80386, part 2: Memory addressing modes

Raymond ChenJanuary 21, 2019Jan 21, 201901/21/19

All of the memory addressing mode demonstrations will be some form of this instruction: which stores a zero somewhere. In practice, the registers used to calculate effective addresses will be 32-bit registers.¹ All the addressing modes look like where the size specifies the number of bytes being accessed, and the something specifies wh

The Intel 80386, part 1: Introduction

Raymond ChenJanuary 20, 2019Jan 20, 201901/20/19

Windows NT stopped supporting the Intel 80386 processor with Windows 4.0, which raised the minimum requirements to an Intel 80486. Therefore, the Intel 80386 technically falls into the category of "processor that Windows once supported but no longer does." This series focuses on the portion of the x86 instruction set available on an 80386, alt