Can you get 80K out of your 48K ZX Spectrum with just one extra chip?? 

This took me back... I upgraded my 16k Spectrum in 1983 to 80k with 8 4164 DRAM chips and a tiny amount of glue logic (digital hardware design was also my day job back then). It worked very well. Also, I compliment you on your explanations of how the addressing of the RAM is handled along with the other hardware. Very well done.

Excellent video HLD! I think I can now, safely say, I almost, very nearly, just about, understand how the Spectrums memory works. One more viewing and im sure I'll have it commited to memory (whichever bank it's in). Well done and looking forward to the sequal.

Ha! You had me. The whole time I'm yelling "but they're likely bad!". ;)

I always laugh when so many RU-vidrs test their upper RAM in a chip tester and discover they are all bad. I wonder why Sinclair didn't do this mod and release it as another model.

I'm hoping the next video is a 128k upgrade for a 48k speccy mod. Something I've always wanted to figure out but have been too lazy to look into

My Romanian clone of the 48K Spectrum had 80KB of RAM from the factory. Due to it having a AY-3 sound chip, it was compatible with a lot of 128K games and enhancements.

I made a similar mod to this for my spectrum back in the 80s. Similar result as well: the half bad chips were indeed half bad.

its a pity they didnt have a /RAMCS line for the upper RAM on the edge connector, then any memory expansions would be easier and could be external ,,...theres a spare 'finger' which couldve been used, and still can if you mod it

For me (at first sight) it seems similar to ZX Spectrum clone named Didaktik Gama (80 KB) from Czechoslovak rep. (actually from Slovak repubic).

As others have pointed out, this will only work correctly if your stack pointer is NOT in the bank switched area of RAM, which means the BASIC program must be in the lower 16K of RAM. As I understand it, this means that BASIC won't be able to directly access the upper 64K (two banks of 32K) for BASIC programs. Also, any machine code routine which may be running in the upper 32K would not be able to use the other 32K directly since switching to it would stop the CPU from accessing the machine code program which would cause a crash. The best use you could probably make of the upper banks would be as a "RAM drive" so that programs of up to the free memory size can be transferred out of one of the high memory areas into the lower 16K, replacing the program which was there. A completely machine-code program may be able to make use of the upper two banks by loading one part of the program into one bank and another part of the program into a different bank and then using a "handoff" routine located inside the lower 16K to perform both the bank switching and the jump into the "second half" of the machine code program. With some clever programming and positioning of code, it may be possible to load what is effectively one program into two different banks of memory, and have a conditional which either jumps past the bank switching or not, depending upon the outcome of a test. Done right, this would save space and prevent crashes, but it's a hassle and is not something which would be supported by any assembler in existence. For example, bank 0 has a test, a jump to the skip location if the test succeeds, the bank switching code if the test fails, then the code which is skipped to if the test succeeds. In bank 1, at the exact same location as the code for if the test succeeded, is code to be run if the test failed. If the test succeeds, the bank switching code is skipped and the "test succeeded" code gets run. If the test fails, the bank switching code gets run and as soon as the last instruction of the bank switching executes, bank 1 becomes active and the next instruction in there is the first instruction of the "test failed" code. All of which is a long, long way of saying that using this extra RAM bank is a hassle and requires some decidedly non-trivial programming tricks. All the same, it's enjoyable to see an old computer used in a new and innovative way. EDIT: Or replace the ROM BASIC with your own version which is bank-switching aware. But that's also non-trivial.

Hi. Learning a lot from your videos. Why your logic analyzer wasn't capable to detect each time the logic was triggered? spectrum clock works at 3.5 Mhz and the data analyzer according to your video goes to 24 Mhz, then why you mentioned that it was not capable enough and that you would need ones that goes to 100 o 200 Mhz!. Thank you

Nice video, I liked your explanation of the upper ram multiplexing, something I've always avoided trying to grok I'm glad you addressed the burning question at the end, as that was on my mind the whole way though.

Holy shit... All of this makes perfect easy sense... Thanks for being such a nerd and doing this work for us mate... Honestly. Wish I'd been able to stay doing anything for more than a few years in my life. (but then other times I'm proud of my screwed up life with adhd)

Great video, and solid explanations the whole way. Will you be doning something similar for how the speccy got 128kb? 😊

That ITT chip could be original, Sinclair would mix up chips in production, so you easily could have had the end of one tube in production, and the next tube grabbed was the ITT one, or that ITT chip was the last, and then the assembler started with a new tube after putting the last one in.

Can you get 128kb ram chips? I need at least 2 and possibly 100! For sam she's 6mhz z80b

Intel do the same with their chips nowadays. The ones with all cores working are intel 10 ( I`m not an expert ) and the ones with less cores working sell as lower chips.

I wish I understood all of this. Analog circuits I can understand, things such as amplifiers, power supplies, tape recording or whatever ... easy. Digital circuits on the other hand ... it's like an alien language to me, allways has been and probably allways will be.

Absolutely brilliant! A crystal clear explanation, thank you.

BRILLIANT! Absolutely brilliant video! It would be nice a small circuit board to plug it directly onto the Speccy motherboard and then pop on int all the involved chips, to avoid this mess of wires... Unfortunately I'm not aware of software to take advantage of this 80K trick. Maybe some utilities and multilevel loading games could have used this feature. This is where the Speccy 128K came handy.

I remember reading a Russian guy's blog where he supposable upgraded his ZX to 1mb. No idea if it worked or not, or how useful the extra 1mb would be.

What about 64kb of fast dual ported video ram?

There was apparently a hardware hack to add some kind of memory mapper on the 48k to allow you to run CP/M from tape... no idea if any of that survived.

Intresting :D ofcourse a flipflop to keep the state, bank switching was and is awesome. might reuse this knowledge for my own computer builds :D is there known software that can make use of this extra ram?

Great video! I'll build it! You're using OUT to switch banks, shouldn't be IORQ instead of MREQ?

you didnt explain the difference with OKI rams, for those that want to know, basically it swaps the order of the selection of A14 and the ?? bits in the upper multiplexer chip 2 ... one brand has a bad column address, the other a bad row, cant remember which for sure but think the TI ones are bad column

problem with this 'expansion' is that any running program must be below 32767 , as with a 16k machine, or you be switching the stacks which can cause issues!

I had an 80k Spectrum back in the '80's. Didn't really get much use out of it though.

Could you make a technical description of the spectra interface please ? Think she has 32kb of video ram

After Uncle Clive's supply of those cheap memory chips exhausted he used the normal real ones, no longer mattered the PCB links were and then Paging was born. Easy to do, but what was the point, no games were written that used it.

At some point I will see how to work this on issues 3 and 4. I guess it would be way harder on issues 5 and 6.

Don't keep us in suspense! Who won the Chaos battle?

The reason why the computer restarts is because the stack pointer points to &FFxx, i.e. in the upper memory area. As soon as you switch the memory area, the stack is undefined and the computer crashes and restarts. The stack is then initialized. Therefore the stack must be secured or the stack must be moved to the lower area. Otherwise the computer will not run properly because the stack will not be consistent when the bank is switched. I checked, the 128k does exactly that, it moves the stack to the unbanked memory area.

I knew about the half-bad RAM back in the day, and often wondered if it would be possible to page between the two sides of the RAM (if you put in memory that was good) in software. After all those jumpers are basically a manual "paging" technique. And now I know.

Wow, really great explanation of this hack. Thank you for sharing. Pretty much interesting ❤

So cool. The demo @32:00 tops it off.

excelent explanation. Really good.

10:11 Just had one of those outside of the litter box thought's ... Can you add a GameBoy Sound Amplifier / Speaker to a Speccy

Damn, speccy had some F up memory organization, but given how sir Clive squeezed every last penny out of it, its not surprising.

Have you ever tried to replace the 4532 memory chips with 41256 ram chips ?

🌝

7:25 seeing c64 on a speccy is crazy!!