Tamago Memory Analysis (Or, Look, I Can Count!)

Having not made any progress trying to identify the Tamago’s microcontroller by comparing its chip to other decapping photos, I’m going to try to identify some of the features of the chip.

To start, it has 139 pins. This might not mean a lot, because if it’s a custom chip/ASIC, it probably has more pins than the microcontroller, but at least rules out anything with more than 139 pins. There’s also 5 squares that look like pins at the bottom right, inside the chip. Maybe these are for programming the external PROM on the Tamagotchi board, interacting with figures, or (dare I dream) JTAG.

Looking at the memory, I suspect block 1 is SRAM, because the cells are very clean and square. Counting the cells, it looks like each cell is  128 x 80 = 1.25 kB x 2 blocks = 2.5 kB of RAM. This seems reasonable for a smaller microcontroller.

Block 2 is quite blurred in the photo, but the chaining pattern every eight cells makes me suspect it’s mask ROM. The cell is 16 bytes blocks by 24 bit blocks = 384 bytes, a very odd number  of bytes indeed.

I think block 3 is also mask ROM, the chaining is much clearer on  it. It looks like its also 16 x 24 = 384 bytes.

Block 4 is a bit odd, it looks like it has cells the same size as the mask ROM’s, but doesn’t have the chaining. Maybe it’s mask ROM and the chaining isn’t showing on the picture, or maybe it’s some other type of ROM (PROM? Flash?).  Anyhow, I can count the number of bits in this one too. 8 bytes by 64 (very blurry, so I might be a bit off) gives 512 , which is a nice round number.

So that’s a total of 1.25k of ROM if it really is all ROM, which I guess is a reasonable number, but it seems odd that it’s all split like that. Some theories:

- There’s some sort of cost/ space savings in doing it that way, but it’s all code/bootloader/what ever else ROM is used for

- There’s two ‘lite figure’ games included in the Tamago, perhaps they have to be read off of mask ROM like other figure games. So maybe each of the 384 byte ROMs is a lite game (seems small, but game logic is handed by the Tamago, not the figure, so it would just have to contain the very minimal images used by the lite games).

- I’m miscounting (maybe what seems like a cell to me is actually a block of 8 cells, etc.)

If I am counting correctly, only 1.25 kB of ROM means the code is almost certainly on the 32 kB external PROM, which is good news with regards to dumping code.

Still doesn’t give much of an idea of what the microcontroller is, but narrows it down a bit.

 

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