Interface 16 KB ROM + 8 KB RAM with 8051

8051 has separate program (\overline{PSEN}) and data (\overline{RD}/\overline{WR}) spaces, each 64KB. Typically ROM as program memory and RAM as data memory.

Option 1: Both as program/data expansion using external memory (most exam expects unified external):

• 16 KB ROM (e.g., 27128): Needs 14 address lines A0-A13. Connected to P0 (AD0-AD7 latched) + P2.0-P2.5 (A8-A13). \overline{PSEN} or \overline{RD} as OE. Address range example if starting at 0000H:
  0000H – 3FFFH (16384 bytes) for 16KB.
  Next free from 4000H.
• 8 KB RAM (e.g., 6264): Needs 13 lines A0-A12. If contiguous after ROM:
  4000H – 5FFFH (8192 bytes) for 8KB.
  Using \overline{RD} and \overline{WR} for OE/WE. Chip select via decoder (74LS138) or simple linear P2.6/P2.7.

Full decoding example via 74LS138 (A15-A13):

• ROM CS = Y0 → 0000H-1FFFH for 8K? But for 16K, use A14=0 block. Simpler: A15=0 for lower 32K. Many textbooks show:
  ROM 16K: 0000H–3FFFH ( \overline{PSEN} ), RAM 8K: 8000H–9FFFH (using P2.7=1) to avoid overlap with different spaces. Because program and data spaces are separate, same address can be reused; the range is logical. If both as data memory (MOVX), same ranges but using \overline{RD}/\overline{WR}.

Hardware: Port 0 via 74LS373 latched by ALE provides A0-A7, Port 2 provides A8-A15, \overline{EA}=1 for internal + external, or 0 for only external. Connect ROM OE to \overline{PSEN} (program) or RAM OE to \overline{RD}.

Address map summary (one common textbook arrangement):

• 16KB ROM: 0000H to 3FFFH
• 8KB RAM: 4000H to 5FFFH (or 0000H-1FFFH in data space if separate buses - then no conflict)
  Calculation: 16KB=2^14 → last = start+3FFFH; 8KB=2^13 → last = start+1FFFH.
  If you place RAM at 0000H data space, its range is 0000H-1FFFH distinct from program ROM.

Diagram: P0→373→A0-A7→ROM/RAM, P2→A8-A15, ALE→373 LE, \overline{PSEN}→ROM OE, \overline{RD}/\overline{WR}→RAM OE/WE, CS decoded from high address lines.

Any contiguous non-overlapping 14-bit/13-bit block is acceptable if decoding explained.