Memory Interfacing with the 8088/8086/80286 Processor — Part 3 (8086 and 80286 Processors)

In our continuing exploration of memory interfacing in microprocessors, we now shift focus to the intricacies involved when interfacing with processors like the 8086 and 80286. Unlike the 8088’s 8-bit data bus, these processors feature a 16-bit data bus, introducing new considerations for memory interfacing. This article breaks down the complexities and strategies for effectively interfacing memory with these more advanced processors.

Understanding 8086 and 80286 Processor Architectures

The 8086 and 80286 processors are notable for their 16-bit data buses and expansive addressable memory spaces — 1MB and 16MB, respectively, thanks to their 20-bit and 24-bit address buses. These capabilities significantly enhance the data handling and memory addressing capabilities over their 8-bit counterparts. The larger data bus means that memory interfacing must accommodate the simultaneous transmission of 16 bits (2 bytes) of data, necessitating a strategic approach to memory organization.

Byte-Organized Memory and 16-Bit Data Bus

Despite the 16-bit bus, memory units are typically byte-organized, storing 8 bits per location. This discrepancy requires that memory be accessed in a manner that aligns with the processor’s data bus width. For effective interfacing, memory is often divided into even and odd banks, allowing for seamless 16-bit data transfers. This division ensures that each part of a 16-bit data word can be stored or retrieved from memory in a single operation, optimizing performance and efficiency.

Interfacing Strategy: Even and Odd Banks

The division of memory into even and odd banks is crucial for the 8086 and 80286 processors. This setup aligns with the processors’ ability to handle 16-bit data, where the even bank is accessed through the lower half of the data bus (D0-D7), and the odd bank through the upper half (D8-D15). The address lines and control signals, specifically A0 for the even bank and the Bus High Enable (BHE) signal for the odd bank, are pivotal in this organization. This methodology ensures that data is evenly distributed and that both halves of the data bus are utilized effectively.

Practical Example: Interfacing ROM and RAM with 8086

Let’s consider a scenario where we need to interface 4KB of ROM and 8KB of RAM with an 8086 processor.

Given the processor’s 16-bit data bus, we must approach this with our even/odd bank strategy:

• ROM Interfacing: To accommodate 4KB of ROM, we divide it into two 2KB banks (even and odd). These banks are mapped to start from address 0000H, utilizing address lines A1-A11 for internal addressing and A0/BHE for bank selection.

• RAM Interfacing: Similarly, 8KB of RAM is divided into four 2KB banks. Starting from address 0800H, each bank is assigned to either the even or odd category, with addressing handled in the same manner as ROM.

Address Decoding and Bank Selection

The decoding logic plays a vital role in selecting the appropriate memory bank based on the current operation. Using a combination of address lines and control signals, the system can determine whether to access even or odd banks and whether the operation involves the ROM or RAM. This setup allows for flexible memory management, supporting a range of operations from simple byte reads/writes to more complex 16-bit transfers.

Conclusion

Memory interfacing with 8086 and 80286 processors introduces complexities not present with the 8088, primarily due to the wider data bus. By dividing memory into even and odd banks and utilizing precise decoding logic, we can efficiently manage memory operations, ensuring that the capabilities of these processors are fully leveraged. This strategy underscores the importance of a thoughtful approach to memory architecture in system design, enabling the construction of high-performance, efficient computing systems.