Using a single 74HC00 would be insufficient to make a decoder with the address mapping of all desired devices.
Instead, a 74HCT138 is to be used as it provides 8 outputs that could be used as a chip-select signal for more devices.
Connecting A15, A14, and A13 of the HD6309 to the A0, A1, and A2 inputs of the 74HC138.
Connecting the three enable inputs (G1, G2A, and G2B) of the 74HC138 as follows:
G1 to VCC (logic high)
G2A and G2B to ground (logic low)
Connecting the eight outputs (Y0-Y7) of the 74HC138 to the CS (chip select) inputs of the RAM, ROM, and UART chips as follows:
Y0 to the CS input of the first RAM chip
Y1 to the CS input of the second RAM chip (for future expansion)
Using a single 74HC00 would be insufficient to make a decoder with the address mapping of all desired devices. Instead, a 74HCT138 is to be used as it provides 8 outputs that could be used as a chip-select signal for more devices.