4×3 Keypad → 7-Segment Display (8086 + 8255 PPI)

An 8086 assembly project that scans a 4×3 matrix keypad through the 8255 Programmable Peripheral Interface (PPI) and drives a common-cathode 7-segment display to show the pressed digit. The 8255 is address-decoded with a 74LS138 so it sits at a fixed I/O range, and the whole system is simulated in Proteus.

Hardware

Component	Role
Intel 8086	Sends scan codes to columns and reads back rows; drives the 7-segment pattern
8255 PPI	Port A → display, Port B → keypad columns (output), Port C lower → keypad rows (input)
74LS138 (3→8 decoder)	Address-decodes the 8255 so its ports map to fixed I/O addresses
4×3 matrix keypad	3 columns driven, 4 rows sensed
Common-cathode 7-segment display	Driven directly by Port A

8255 port map

The 74LS138 places the 8255 at I/O addresses 68h and above:

Port	Address	Direction	Connected to
Port A	68h	output	7-segment cathodes (a..g)
Port B	6Ah	output	Keypad column drives (one-hot)
Port C (lower)	6Ch	input	Keypad row sense lines (PC0..PC3)
Control word	6Eh	—	88h — A out, B out, C upper in

How it works

1. Init — write 88h to the 8255 control register (Mode 0; Port A output, Port B output, Port C upper input).
2. Scan loop — for each column 1..3, drive a one-hot pattern on Port B (100b, 010b, 001b), then read Port C and mask the low nibble. A non-zero result means a key in that column is pressed.
3. Resolve the key — jump to per-column handler (column1 / column2 / column3), which compares the row pattern (0001b, 0010b, 0100b, 1000b) to identify the exact row → digit.
4. Display — write the matching 7-segment pattern to Port A and resume scanning.

Keypad → 7-segment pattern map

Key	Column drive (PB)	Row sense (PC)	Pattern on PA (gfedcba)	Hex	Shown
1	100b	0001b	0000110	06h	1
4	100b	0010b	1100110	66h	4
7	100b	0100b	0000111	07h	7
2	010b	0001b	1011011	5Bh	2
5	010b	0010b	1101101	6Dh	5
8	010b	0100b	1111111	7Fh	8
0	010b	1000b	0111111	3Fh	0
3	001b	0001b	1001111	4Fh	3
6	001b	0010b	1111101	7Dh	6
9	001b	0100b	1101111	6Fh	9

The keypad's * and # positions are intentionally unmapped — the scanner falls through and continues polling.

Files

.
├── Assembly_code.asm        # 8086 source (MASM32 syntax)
├── Hardware_project.pdsprj  # Proteus simulation (8086 + 8255 + 74LS138 + keypad + display)
├── Report.docx              # Written report
├── README.md
└── LICENSE

Run the simulation

1. Install Proteus (8.x).
2. Open Hardware_project.pdsprj — the assembled firmware is already bundled inside the project (Proteus stores it as 8086/Debug/Debug.exe).
3. Hit ▶ Run and click keypad buttons — the digit appears on the 7-segment display.

Want to rebuild from source? Install MASM32, then open the source pane in Proteus VSM Studio and edit main.asm (or just replace the bundled file with Assembly_code.asm from the repo root — they are identical). Proteus' 8086 model also requires Internal Memory Size = 0x10000 in its properties.

Course context

Built for the Microprocessors course at PSUT. Demonstrates I/O port handling on the 8086, address decoding with the 74LS138, matrix-keypad scanning, and 7-segment encoding; without using any library calls.

Author

Leen Almousa — github.com/leenalmousa

License

Released under the MIT License.