ALE (Address Latch Enable) and Its Role in Address/Data Bus De-multiplexing in 8085

What is ALE?

ALE (Address Latch Enable) is a control signal generated by the 8085 microprocessor during the first clock cycle (T1) of every machine cycle. It is an active-high output signal that indicates the lower 8-bit address (A0-A7) is present on the multiplexed Address/Data bus (AD0-AD7).

In simple terms: ALE is a signal that tells external hardware "right now the bus carries an address, latch it before it changes to data."

Why is De-multiplexing Needed?

The 8085 microprocessor has only 40 pins. To reduce pin count, Intel multiplexed the lower 8 address lines (A0-A7) with the 8 data lines (D0-D7) on the same physical pins, called the AD0-AD7 bus.

• The upper 8 address bits (A8-A15) are available on dedicated pins and do not need de-multiplexing.
• The lower 8 address bits (A0-A7) share pins with data, so they must be separated (de-multiplexed) using ALE.

Role of ALE in De-multiplexing

Step-by-Step Process:

1. At T1: ALE goes HIGH. The lower 8-bit address (A0-A7) is placed on the AD0-AD7 bus.
2. Latch captures address: The HIGH-to-LOW transition of ALE triggers an external latch (e.g., 74LS373) to capture and hold the lower 8-bit address.
3. After T1: ALE goes LOW. The AD0-AD7 bus now carries data (D0-D7) for read/write operations.
4. Latch holds address: The latch continues to output the stable lower 8-bit address (A0-A7) throughout the remaining machine cycle.
5. Full 16-bit address is now available: Upper 8 bits (A8-A15) directly from the processor + Lower 8 bits (A0-A7) from the latch output.

Diagram: Address/Data Bus De-multiplexing Using ALE

        8085 Microprocessor
   +---------------------------+
   |                           |
   |  A8 - A15  |--------------|---------> A8-A15 (Upper Address Bus)
   |            |              |
   |  AD0 - AD7 |----+---------+---------> D0-D7  (Data Bus)
   |            |    |         |
   |    ALE     |----|--+      |
   |            |    |  |      |
   +---------------------------+
                    |  |
                    |  | ALE (HIGH at T1, latches address)
                    |  |
                    v  v
              +------------+
              |  Latch     |   (e.g., 74LS373)
              |  74LS373   |
              |            |
              | G (Enable) |<---- ALE
              +------------+
                    |
                    v
               A0 - A7  (Lower Address Bus, stable throughout)
                    |
                    v
   +----------------+------------------+
   |         Full 16-bit Address Bus   |
   |         A0-A15 to Memory/I/O      |
   +-----------------------------------+

Timing Diagram:

CLK  :  |--T1--|--T2--|--T3--|

ALE  :  |HIGH  |LOW   |LOW   |

AD0-7:  |ADDR  |DATA  |DATA  |
        (A0-A7)(D0-D7)(D0-D7)

Latch:  |Captures A0-A7 on falling edge of ALE|
        |Holds A0-A7 stable for entire cycle  |

Summary

Conclusion: ALE is essential in the 8085 because the multiplexed AD0-AD7 bus serves dual purpose. Without ALE and an external latch, it would be impossible to distinguish between address and data on those shared lines, making memory and I/O communication unreliable.

Flags in 8085 Microprocessor

What is a Flag?

A flag is a single-bit flip-flop (1-bit register) that indicates the status of the accumulator or other registers after the completion of an arithmetic or logical operation. These flip-flops are set (1) or reset (0) according to the data condition of the result stored in the accumulator.

The flag register (also called the status register or F register) is an 8-bit register, but only 5 bits are used as flags in the 8085 microprocessor.

The 5 Flags of 8085 Microprocessor

The flag register layout is as follows:

1. Sign Flag (S) -- Bit 7

• Set to 1 if the MSB (bit 7) of the result is 1, indicating the result is negative.
• Reset to 0 if the result is positive (MSB = 0).
• Used in signed number arithmetic.

Example: If result = 10000001 (negative in 2's complement), S = 1.

2. Zero Flag (Z) -- Bit 6

• Set to 1 if the result of the operation is zero.
• Reset to 0 if the result is non-zero.

Example: If 05H - 05H = 00H, then Z = 1.

3. Auxiliary Carry Flag (AC) -- Bit 4

• Set to 1 if there is a carry out from bit 3 to bit 4 (i.e., carry from the lower nibble to the upper nibble).
• Primarily used in BCD (Binary Coded Decimal) arithmetic operations.

Example: If lower nibble addition produces a carry to bit 4, AC = 1.

4. Parity Flag (P) -- Bit 2

• Set to 1 if the result contains an even number of 1s (even parity).
• Reset to 0 if the result contains an odd number of 1s (odd parity).

Example: If result = 00000011 (two 1s -- even), then P = 1.

5. Carry Flag (CY) -- Bit 0

• Set to 1 if there is a carry out from the MSB (bit 7) during addition, or a borrow during subtraction.
• Reset to 0 if there is no carry or borrow.

Example: If FFH + 01H = 100H (carry out), then CY = 1.

Summary Table

These five flags together help the programmer make conditional decisions (using conditional jump instructions) based on the outcome of arithmetic and logical operations.

Functional Units of 80386 Microprocessor

Introduction

The 80386 is a 32-bit microprocessor. To achieve high performance through parallelism, it is divided into several independent functional units that can operate simultaneously.

Functional Units of 80386

The 80386 microprocessor is internally divided into six functional units:

1. Bus Interface Unit (BIU)

• Manages all data and address transfers on the external buses (data bus, address bus, control bus).
• Handles communication between the processor and external memory or I/O devices.
• Generates the 32-bit physical address for memory access.
• Controls bus cycles and arbitration.

2. Code Prefetch Unit (CPU / Prefetch Unit)

• Fetches instruction bytes from memory in advance (before they are needed).
• Stores prefetched bytes in a 16-byte prefetch queue.
• Keeps the instruction pipeline filled so the processor does not have to wait for instruction fetch.
• Works in parallel with instruction decoding and execution.

3. Instruction Decode Unit

• Takes instruction bytes from the prefetch queue and decodes them into internal micro-operations.
• Maintains a queue of 3 decoded instructions ready for execution.
• Decoding in advance reduces delays during execution.

4. Execution Unit (EU)

• Carries out the actual arithmetic and logical operations on data.
• Contains the 32-bit ALU (Arithmetic Logic Unit) for integer operations.
• Accesses the general-purpose registers (EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP).
• Executes decoded instructions passed from the Instruction Decode Unit.

5. Segmentation Unit

• Handles memory segmentation by converting logical addresses (segment:offset) into linear addresses.
• Uses segment registers (CS, DS, SS, ES, FS, GS) and descriptor tables.
• Provides memory protection by checking segment limits and access rights.

6. Paging Unit

• Converts linear addresses (produced by the Segmentation Unit) into physical addresses.
• Implements demand paging using page tables and a Translation Lookaside Buffer (TLB) for fast address translation.
• Supports a 4 KB page size.
• Only active when paging is enabled (PG bit set in CR0).

Summary Table

Conclusion

These six units work in parallel (pipelined), so while one instruction is being executed, the next is being decoded and the one after that is being prefetched. This parallel operation significantly increases the throughput and performance of the 80386 processor.