Search found 6 matches

by VIP Quality Post
Mon May 13, 2019 6:47 pm
Forum: SNESdev
Topic: Purpose of MAD-1 and 74139 logic for single ROM boards?
Replies: 23
Views: 23913

Re: Purpose of MAD-1 and 74139 logic for single ROM boards?

Hi again- I disappeared because I was busy with other work, but I've had some free time and am returning to the project. I found a really helpful resource: http://randombazz.blogspot.com/2011/09/snes-memory-mappingaddress-decoding.html For me, this really helped me to understand the 74139/138. It de...
by VIP Quality Post
Thu Jan 24, 2019 9:17 am
Forum: SNESdev
Topic: Purpose of MAD-1 and 74139 logic for single ROM boards?
Replies: 23
Views: 23913

Re: Purpose of MAD-1 and 74139 logic for single ROM boards?

I understand the purpose of the chip now. I think I just had some core misunderstandings on the edge connector roles. Regarding the role of specific address pins in enabling/disabling the ROM and RAM, do you have any ideas about why those pins (A21/A22) are used for controlling the decoder? Addition...
by VIP Quality Post
Thu Jan 24, 2019 8:42 am
Forum: SNESdev
Topic: Purpose of MAD-1 and 74139 logic for single ROM boards?
Replies: 23
Views: 23913

Re: Purpose of MAD-1 and 74139 logic for single ROM boards?

Okay, so since there is no pin dedicated to enabling cartridge RAM, we use the decoder to enable RAM when pointing at external WRAM space, and this is done on A21 and A22? Also, what do you mean when writing to 'conventionally "ROM" space'- are you referring to the cartridge, or the ROM itself? I wa...
by VIP Quality Post
Thu Jan 24, 2019 8:23 am
Forum: SNESdev
Topic: Purpose of MAD-1 and 74139 logic for single ROM boards?
Replies: 23
Views: 23913

Re: Purpose of MAD-1 and 74139 logic for single ROM boards?

As a note, I see a lot of this diagram passed around, originating from warosu, some online board https://i.warosu.org/data/vr/img/0037/07/1483158364085.gif The connections for the 74139 kind of make sense, with /WR used to enable writes on the RAM, and /RD for /OE on ROM and RAM. However, I've seen ...
by VIP Quality Post
Thu Jan 24, 2019 8:03 am
Forum: SNESdev
Topic: Purpose of MAD-1 and 74139 logic for single ROM boards?
Replies: 23
Views: 23913

Purpose of MAD-1 and 74139 logic for single ROM boards?

Hello everyone! Longtime lurker, first time post. I am working on reverse engineering a SNES cartridge as a personal project to learn a bit more about how the system works. For the most part, I understand how the cartridges work, with the exception of the MAD-1 chip/74139 seen on earlier boards. Thi...