After I was able to get my 6502 computer working, I wanted to add sound. I decided I was going to have a dedicated sound card so that the 6502 microprocessor wouldn't have to do much work. To do this I decided to have an input of 8 bits to control the tone. I had already used 11 out of the 16 outputs and decided I wanted to create a memory mapped output designed just for the sound card. Memory mapped means that instead of writing to the ram (memory) it would write to the output. This is commonly used in old game consoles for inputs and outputs such as controllers. To make this memory mapped output I used an 8 bit register to store the value and a magnitude comparer chip. This chip would tell me if the address value was equal to a certain value. At the time I didn't have any chip that would do this for me so I used simple logic gates such as an and gate and an or gate. Because I had limited room on the bread board I made a short cut where I compared only the first 8 values of the address to see if they were what I wanted. This will work most of the time but I could accidentally write to the output at higher addresses. For example, 0000 0000 0000 0000 would write to the output but so would 0000 0001 0000 0000. This works most of the time but sometimes my programs will reach that high and lead to random sounds. I plan to redesign this output so it doesn't have this problem.
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