8085 Microprocessor Address Bus Width Explained

The address bus is a group of lines used by the microprocessor to send addresses to the memory or I/O devices. The width of the address bus determines the total number of unique memory locations that the microprocessor can access.

For the 8085 microprocessor, a widely studied 8-bit microprocessor, the address bus has a specific width. This width dictates its memory addressing capability.

Understanding 8085 Address Bus

• The 8085 microprocessor uses an address bus to specify the location of data it wants to read from or write to in memory, or the I/O device it wants to communicate with.
• The number of lines in the address bus directly corresponds to the number of bits in an address.
• More address lines mean a larger address space, allowing the microprocessor to access more memory locations.

8085 Address Bus Width

The 8085 microprocessor has a 16-bit address bus. This means that there are 16 physical lines dedicated to carrying address information.

Each line in the address bus can represent either a logic 0 or a logic 1. With 16 lines, the total number of unique addresses that can be generated is \(2^{16}\).

Calculating 8085 Memory Capacity

The number of unique addresses that the microprocessor can generate determines the maximum memory size it can directly address.

Maximum memory capacity = \(2^{\text{Address Bus Width}}\)

For the 8085 microprocessor with a 16-bit address bus:

Maximum memory capacity = \(2^{16}\) locations

We know that \(2^{10} = 1024 \approx 1\text{ KB}\). Therefore, \(2^{16} = 2^{10} \times 2^6 = 1\text{ KB} \times 64 = 64\text{ KB}\).

So, the 8085 microprocessor can directly address up to 64 KB of memory.

It's important to distinguish this from the data bus width. The 8085 is an 8-bit microprocessor, meaning its data bus width is 8 bits. The data bus carries the actual data being transferred, while the address bus carries the location of that data.

Comparing Options

Let's look at the given options for the address bus width of the 8085 microprocessor:

1. 32 bit: This is common for more advanced microprocessors like the 80386 or later, not the 8085.
2. 8 bit: This is the width of the data bus for the 8085, not the address bus.
3. 16 bit: This is the correct width of the address bus for the 8085 microprocessor.
4. 64 bit: This width is found in modern high-performance processors, not the 8085.

Based on the structure and specifications of the 8085 microprocessor, its address bus is 16 bits wide.

Revision Table: 8085 Microprocessor Basics

Additional Information: 8085 Bus Multiplexing

It's worth noting that the 8085 microprocessor uses a multiplexed address/data bus. This means that some of the lower-order address lines are time-shared with the data lines.

• The lower 8 bits of the address (A0-A7) and the 8 bits of data (D0-D7) share the same physical pins (AD0-AD7).
• The higher 8 bits of the address (A8-A15) have dedicated pins.
• An external latch is required to demultiplex the lower-order address from the data lines during a memory or I/O operation.
• This multiplexing was done to reduce the number of pins required on the IC package, allowing the 8085 to be a 40-pin chip.

Even though the lower address and data lines are multiplexed on the same physical pins, the logical address bus is considered 16 bits wide because the microprocessor generates a full 16-bit address during addressing cycles.