With ROM, it skips the "real" A0 and A1. This is because each address on the address bus is sent to all 4 ROM chips. Because there are 4 8-bit ROM chips, each address requested from ROM returns 32 bits. But as we know, each single byte (each 8 bits) has an address, not each 32 bits. So by skipping the first 2 address bits, the physical address sent to the ROM changes each 4 bytes instead of each byte. These missing address bits are probably used elsewhere to grab a specific 8-bit portion of the 32-bit data returned from ROM.
"real" A19, which logically should be A17 to the ROM chips, the one that would let me expand ROM to 1024k, is connected on my IIfx SIMM to /WE (write enable) on each ROM chip. This line must always be active high to enable output from the ROM chips. I guess it's possible you could use it as a third chip select input. All 3 /CS, /OE, and /WE must be low to enable the output of the chip. This is doubtful however because this line is NOT connected to the onboard ROM at all.
Then I took my trusty voltmeter to this pin. ZERO volts. That was unexpected, but it gives me a little more confidence in A19 working with ROM.
I will admit that this confuses me a little. I guess we'll just have to see what happens when I get these parts in. I ordered ROMs that use the extra bit and as such are twice as big, totaling to 1024kB. It won't be for a while, but I'll let you know. A IIci running Mac LC II/III ROM would be quite the conversation piece, even though it doesn't seem too likely. I will give it a shot though!
"real" A19, which logically should be A17 to the ROM chips, the one that would let me expand ROM to 1024k, is connected on my IIfx SIMM to /WE (write enable) on each ROM chip. This line must always be active high to enable output from the ROM chips. I guess it's possible you could use it as a third chip select input. All 3 /CS, /OE, and /WE must be low to enable the output of the chip. This is doubtful however because this line is NOT connected to the onboard ROM at all.
Then I took my trusty voltmeter to this pin. ZERO volts. That was unexpected, but it gives me a little more confidence in A19 working with ROM.
I will admit that this confuses me a little. I guess we'll just have to see what happens when I get these parts in. I ordered ROMs that use the extra bit and as such are twice as big, totaling to 1024kB. It won't be for a while, but I'll let you know. A IIci running Mac LC II/III ROM would be quite the conversation piece, even though it doesn't seem too likely. I will give it a shot though!