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

Raymond Chen January 31, 2019 Jan 31, 2019 01/31/19

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 ...

The Intel 80386, part 9: Stack frame instructions

Raymond Chen January 30, 2019 Jan 30, 2019 01/30/19

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 Chen January 29, 2019 Jan 29, 2019 01/29/19

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) ...

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

Raymond Chen January 28, 2019 Jan 28, 2019 01/28/19

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 ...

The Intel 80386, part 6: Data transfer instructions

Raymond Chen January 27, 2019 Jan 27, 2019 01/27/19

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 ...

The Intel 80386, part 5: Logical operations

Raymond Chen January 24, 2019 Jan 24, 2019 01/24/19

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 Chen January 23, 2019 Jan 23, 2019 01/23/19

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 ...

The Intel 80386, part 3: Flags and condition codes

Raymond Chen January 22, 2019 Jan 22, 2019 01/22/19

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 Chen January 21, 2019 Jan 21, 2019 01/21/19

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 ...

The Intel 80386, part 1: Introduction

Raymond Chen January 20, 2019 Jan 20, 2019 01/20/19

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, ...