Why not a Static RAM board for 128k Mac?

[JustDaveIII]

I have a silly question. But first, I am grateful for all the work put into the memory expansion board detailed here in another thread. Thank You! I have been designing logic circuits since the RTL / ECL days, modifying motherboards and also do assembly language on multiple platforms. So I know the effort that was needed here.

Now for the silly question. Why not a static ram memory expansion that uses the ROM sockets for address & data lines? 4mb (256kx16) static ram chips are cheap. Not like in the old days when I was buying 1k x 4 2114's for $5 each.

Is it not possible to map some static memory from 128k - 512k by grabbing the address (and extra ones that are not to the ROM sockets) and R/W lines ? Like the Rominator? Static memory has like, 10ns access time vs the 450ns of the ROM and the 200ns access (335ns cycle time) of the 4864 chips. So I don't see that as a hurdle.

So please point me in the right directions so I don’t go down the rabbit hole on my next project. What am I missing?

Why do I want to do this? I have an original Mac128k board. It's been sitting around since late last century. I've added the PiPico vga adapter, my own PS/2 mouse & keyboard adapter, Floppy EMU and more. But I don't want to alter the board as there are very few untouched original 128k boards left. Yet I need more memory for System 6/7 and to play Dark Castle II.

ETA: I've written a lot of 68xxx code for Mac and was Mac / Newton Developer a long time ago.

 

[Builder68]

I have a silly question. But first, I am grateful for all the work put into the memory expansion board detailed here in another thread. Thank You! I have been designing logic circuits since the RTL / ECL days, modifying motherboards and also do assembly language on multiple platforms. So I know the effort that was needed here.

Now for the silly question. Why not a static ram memory expansion that uses the ROM sockets for address & data lines? 4mb (256kx16) static ram chips are cheap. Not like in the old days when I was buying 1k x 4 2114's for $5 each.

Is it not possible to map some static memory from 128k - 512k by grabbing the address (and extra ones that are not to the ROM sockets) and R/W lines ? Like the Rominator? Static memory has like, 10ns access time vs the 450ns of the ROM and the 200ns access (335ns cycle time) of the 4864 chips. So I don't see that as a hurdle.

So please point me in the right directions so I don’t go down the rabbit hole on my next project. What am I missing?

Why do I want to do this? I have an original Mac128k board. It's been sitting around since late last century. I've added the PiPico vga adapter, my own PS/2 mouse & keyboard adapter, Floppy EMU and more. But I don't want to alter the board as there are very few untouched original 128k boards left. Yet I need more memory for System 6/7 and to play Dark Castle II.

ETA: I've written a lot of 68xxx code for Mac and was Mac / Newton Developer a long time ago.

I think IMHO the major hurdle is that mapping continuous system RAM through the ROM sockets is essentially impossible because you cannot isolate the bus or change the address decoding without altering that beautiful, untouched 128K motherboard.

The Mac 128K doesn’t have a dedicated video controller with its own VRAM. Instead, the 68000 CPU and the video/sound hardware share the exact same physical DRAM.

The bus timing is strictly interleaved: One cycle goes to the video/sound circuits to fetch data for the screen, and the next cycle goes to the CPU.

Since the video circuits are hardwired to pull data directly from the onboard DRAM space, any memory you map elsewhere (like the ROM sockets) is invisible to the video hardware.

You couldn't really use that SRAM for the frame buffer since the ROM pins the video address to the top 32 KB of whatever RAM it detects. To make it work, your expansion would have to 'bank-switch' back to the onboard DRAM for video (which is how most old-school accelerators handled it), or you'd have to replicate the Mac's internal decoding so the CPU can find the video buffer at the top of your new SRAM, much like a MacSnap does.