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Using the double-channel UART requires the following:

• Crystal oscillator: 1.8432 MHz for the baud rate generator
• Filtering caps
• additional circuitry

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.

1. Connecting A15, A14, and A13 of the HD6309 to the A0, A1, and A2 inputs of the 74HC138.
2. 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)

3. 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)
• Y2 to the CS input of the ROM chip
• Y3 to the CS input of the UART chip
• Y4-Y7 to be used as extension port CS pins

Hereby discussing memory mapping and address decoding for a system with two RAM chips.

In general, to access multiple RAM chips, we need to ensure that the CPU can distinguish between the different RAM chips based on their respective memory addresses. This is typically done using an address decoder, which takes the address lines from the CPU and activates the appropriate RAM chip based on the address range.

In our specific case, we have two Winbond 24257 RAM chips.
Each chip has 32K of memory and requires 15 address lines (A0-A14) to access all of its memory locations. To address both chips, we need to use the CPU's 16-bit address bus, which provides 64K of address space. This means that we can use the first 32K of the address space to access the first RAM chip and the second 32K to access the second RAM chip.

We discussed several options for address decoding. One option is to use an 8-bit address decoder, such as a 74HCT138, to decode the most significant address bit (A15) and select between the two RAM chips. Another option is to use a combination of logic gates to decode the A15 and A14 address lines and select between the two RAM chips based on their combinations.