Unused space in-between GROM chips

[scanline_]

Hi all!

 

Maybe someone can enlighten me! The TIs three main GROMs installed in the
console are located at addresses 0x0, 0x2000, 0x4000 and are each 6144 bytes.
So there are unused gaps in-between 0x1800-0x1fff, 0x3800-0x3fff and
0x5800-0x5fff.
Now if I look at the bytes from a system GROM dump floating around the internet
most of you surely know (994AGROM.BIN | 24576 bytes) the aforementioned gaps

do not just contain zeros, there seems to be actual data.
Now I'm wondering what is that data? Is it just some random bytes to fill-up the space
(though I'm not sure what that would be good for).

[brain]

Maybe it's just noise on the data bus when teh GROMs were read, as the GROMs probably don't even respond to those addresses, assuming another GROM will do so, and none end up driving the bus.

 

Jim

 

[scanline_]

Aaah, it's that simple. Thanks Jim!

[+mizapf]

No, it's a bit more complicated. The GROMs are PMOS devices with three 2K memory banks. Using an address prefix of 00 selects the first bank; 01 selects the second; 10 selects the third, and 11 selects both second and third. Both OR their data to the outputs.

 

See also https://github.com/mamedev/mame/blob/master/src/devices/machine/tmc0430.cpp

[RXB]

GROMs for SOB (Son Of Board) is a substitute for TI GROMS 0, 1 and 2.

It has built in multi cartridge support.

It has built in disk and hard drive cataloger.

It has built in EA5 and EA3 loading utility.

[+Ksarul]

1 hour ago, RXB said:

GROMs for SOB (Son Of Board) is a substitute for TI GROMS 0, 1 and 2.

It has built in multi cartridge support.

It has built in disk and hard drive cataloger.

It has built in EA5 and EA3 loading utility.

Yes--but those are very different from the TI GROMS, as they fully decode all four 2K segments of each reserved 8K GROM space. Original GROMs would demonstrate the symptoms described by the original poster, whereas the SOB was a later third-party development designed to fully exploit the GROM space--and the data in that formerly garbled extra space provided enough usable space to add the functions you described. RXB and other third-party Extended BASIC variants did the same thing to use the 10K of excess mapped space in the cartridge GROM space. That flexibility exploited by later programmers (including you) is a nice feature of the TI.