Data Types and Addressing Modes of 80386/80386DX Microprocessor

Data types play a crucial role in microprocessor programming. Variables define the types of information that can be saved and the actions that can be done with that information. It is essential to have a good understanding of the data types supported by a microprocessor in order to create efficient and effective code. Different microprocessors may have varying support for data types, and the choice of data type can significantly impact the performance of the code. This article explores the various commonly supported data types in microprocessors, along with their uses and limitations.

Microprocessors utilize basic data types such as bytes, words, and double words. A byte consists of eight consecutive bits, which are numbered from 0 to 7. The smallest and least important bit is called bit zero, and it begins at any logical address.

In microprocessors, a word consists of two consecutive bytes that start at any byte address. A word contains 16 bits, which are numbered from 0 to 15. The smallest and least important bit is called bit 0. In a word, the byte that contains bit 0 is referred to as the low byte, while the byte that contains bit 15 is known as the high byte.

In microprocessors, a double word is created by two sequential words that start at any byte address. A double word consists of 32 bits, which are numbered from 0 to 31. The smallest and least important bit is called bit 0. In a double word, the word that holds bit 0 is referred to as the low word, while the word that holds bit 31 is called the high word.

The 80386DX microprocessor provides support for additional interpretations of the fundamental data types, such as bytes, words, and double words. These additional data types are recognized based on the instruction that refers to the operand. The 80386DX and 80386 microprocessors share the same data types.

Bit: A single-bit quantity refers to a data type that consists of only one bit.

• Bit Field: A collection of up to 32 consecutive bits is called a "bitfield," and it can span a maximum of four bytes.
• Bit String: The Intel386 DX can handle bit strings that are up to 4 gigabits long. This means they can consist of a continuous sequence of up to 4 billion bits.
• Byte: A signed 8-bit quantity represents values from -128 to +127. It uses 8 bits to store the value and can represent both negative and positive numbers within this range. An unsigned byte, on the other hand, is an 8-bit data type that can represent values from 0 to 255. It does not have a sign, so all values are considered positive.
• Integer (Word): A signed 16-bit quantity is a type of data that can store values from -32,768 to +32,767. It uses 16 bits to represent the value and can represent both negative and positive numbers within this range.
• Unsigned Integer (Word): An unsigned 16-bit quantity is a data type that can hold values from 0 to 65,535. It uses 16 bits to store the value and can only represent positive numbers within this range.
• Long Integer (Double Word): A long integer (double word) is a data type that represents a signed 32-bit quantity using a 2's complement representation. It can represent values ranging from -2 to the power of 3 to +2 to the power of 31 - 1. This means it can store both negative and positive numbers within this specified range.
• Unsigned Long Integer (Double Word): An unsigned 32-bit quantity is a data type that can hold values from 0 to 4,294,967,295. It uses 32 bits to store the value and can only represent positive numbers within this range.
• Signed Quad Word: A signed 64-bit quantity is a data type that can represent a wide range of values, both positive and negative. It uses 64 bits to store the value, allowing for larger numbers to be represented compared to smaller data types.
• Unsigned Quad Word: An unsigned 64-bit quantity is a data type that can store only positive numbers within a wide range. It uses 64 bits to hold the value, allowing for larger non-negative values compared to smaller data types.
• Offset: An offset-only quantity of either 16 or 32 bits is used to indirectly refer to another memory location. It serves as a way to specify a distance or displacement from a specific point in memory, rather than directly referencing the target memory location itself.
• Near Pointer: A 32-bit logical address is a type of memory address that consists of 32 bits. It is used to indicate a particular position in memory. On the other hand, a near pointer represents a distance within a segment. It is utilized in either a flat or segmented memory model to point to a particular memory location.
• Far Pointer: A far pointer is a data type that represents a 48-bit logical address, consisting of two components: a 16-bit segment selector and a 32-bit offset. Application programmers utilize far pointers only when the system designers opt for a segmented memory structure. They allow for referencing specific memory locations beyond the limitations of a regular 32-bit address.
• Char: A byte representation means converting an ASCII alphanumeric or control character into a specific encoding. It uses a single byte of data to represent a specific character from the ASCII character set, which includes letters, numbers, and control characters.
• String: A string is a series of consecutive bytes, words, or double words. It can range in size from 1 byte up to 4 gigabytes, accommodating a wide range of data lengths.
• BCD: A byte (unpacked) representation refers to the encoding of decimal digits 0 to 9 using individual bytes. Each digit is represented by a separate byte, allowing for easy manipulation and storage of decimal values in computer systems.
• Packed BCD: A packed byte representation means storing decimal digits from 0 to 9 using nibbles. Each digit is stored in a nibble, which is half a byte, allowing for efficient storage of multiple digits in a compact manner.When the Intel386 DX is combined with an Intel387 DX Numeric Coprocessor, it provides support for various popular floating-point types. These floating-point types enable precise representation and manipulation of decimal numbers with decimal points.
• Floating Point: The Intel387 DX numeric coprocessor supports the representation of signed real numbers using 32-bit, 64-bit, or 80-bit floating-point formats. These formats allow for accurate representation and computation of decimal numbers in computer systems.

Categorizing Addressing Modes 80386/80386DX

To enhance programming flexibility, processors offer various methods to access operands from different locations. The 80386 microprocessor offers multiple addressing modes that facilitate the access of memory and registers. These methods are commonly referred to as addressing modes.

Addressing modes play a crucial role in determining how the processor interprets a memory address or register specified in an instruction. They define the method of accessing and interpreting the specified memory or register. The 80386DX offers a total of 11 addressing modes for specifying operands, which can be classified into three groups:

1. Register operand addressing
2. Immediate operand addressing
3. Memory operand addressing

Register Operand Addressing: In this addressing mode, the instruction specifies a register as the operand.

The operand is the value stored within the register. This method of addressing is extremely fast because it only requires the CPU to access the register, which is located directly on the processor. It is commonly used for arithmetic and logical operations, as well as data transfers between registers. The value can be saved in a 32-bit general register (EDX, ECX, EBX, EAX, EDI, ESI, EBP, or ESP), a 16-bit general register (DX, CX, BX, AX, DI, SI, BP, or SP), or an 8-bit general register (DH, CH, BH, AH, BL, AL, DL, or CL).

Immediate Operand Addressing: In certain instructions, the operand is a constant value embedded within the instruction itself. This addressing mode is particularly useful for operations that require a fixed value as an operand, such as setting a value or comparing it to a constant. In some cases, an instruction can use data from within itself as an operand, which is known as an immediate operand. The length of the operand can be 32, 16, or 8 bits, depending on the instruction.

Memory operand Addressing: In this addressing mode, the instruction includes a memory address as the value. The data stored at that particular memory location is utilized as the value for the instruction.

This mode is used for operations that involve reading from or writing to memory, like transferring data into a register or saving data from a register to memory. The memory operand addressing mode can be divided into different types: direct addressing, indirect addressing, register indirect addressing, base-pointer-indexed addressing, scaled indexed addressing, and relative addressing. Each of these addressing modes has its own specific purposes and use cases.