Hi, qwertykeyboard

In fact it *does* start at $EAEA but you're not seeing that very first cycle because this procedure you're using is slightly misguided: "It does 2 clock cycles before reading off the address bus since 2 clock cycles are needed to complete the NOP instruction." Instead, try reading off the address in the first cycle. Here's why.

With 65xx processors all opcodes are a single byte, and sometimes an instruction will have one or more operand bytes following the opcode. For performance reasons the processor always follows the opcode fetch with a fetch of the following byte, and often this saves time. NOP is one of the exceptions, however, as there are no operand bytes following the opcode. But the extra fetch happens anyway.

So! After having fetched the Rest Vector the CPU goes to $EAEA to find its first instruction. It fetches the byte at $EAEA, which is the opcode. It then fetches the byte at $EAEB while the NOP completes its execution. But this fetch is actually pointless because there's no operand. After the NOP has completed the CPU will go looking for the next opcode... at $EAEB! IOW it fetches that same byte again. This same pattern repeats endlessly, with the address bus incrementing after the 1st, 3rd, 5th cycle and so on. I hope I explained that so it makes sense!

-- Jeff

Current versions of the 74HC138 typically achieve prop times in the 10-15ns range on 5 volts, which is way more than is needed in a system running at a couple of megahertz. For faster units, the 74AC138 routinely achieves single digit prop times on 5 volts. For example, ON Semiconductor rates their 74AC138's tPLH at 6.5ns typical, 9.5ns maximum. My POC V1.1 unit uses a 74AC138 to decode I/O and will boot at 15 MHz if the SCSI host adapter isn't installed.

My NSC data book says a max of 40ns @ 4.5V, 34ns @ 6V for the 74HC138's tPHL (which is longer than tPLH). [Edit: It's 60ns max and 51ns max for 4.5V and 6V @ 125°C. Zowee!! Even the '154 is faster, at 30ns max for 6V, and 39ns max for 4.5V, at 125°C.] The 74AC part is much faster but I'm not very anxious to recommend 74AC to new builders, since the fast slew rates tend to get them in trouble.

Your NSC book is woefully out of date. Please scroll back and read the 74HC138 data sheet I attached to my post.

Current versions of the 74HC138 typically achieve prop times in the 10-15ns range on 5 volts,

Could the 6809 be a viable option as well?

Well... That's one on my list to build a system around but I'll tell you this, the 6502.org forum really helped me get started building my Tower of Eightness WDC65C02 based system, an ongoing project that keeps me out of trouble.