It was a multifaceted problem, and was ultimately a design flaw/oversight rather than someone saying "I think 4kB is enough memory to store all the textures". The problem is less that the cache was small, it's more that Nintendo's plans for how awesome RDRAM and a unified memory architecture didn't pan out.
Problem #1: There was no dedicated video memory. All RAM on the N64 was shared RAM. So framerates tanked if you didn't have most of your stuff in cache. Keep in mind the framebuffer also lived in this unified memory area, so the video chip was already very noisy on the memory bus.
Problem #2: The unified shared system RAM was RDRAM, not SDRAM. And the latency on RDRAM is absolutely terrible. So the already expensive cost of using RAM was compounded.
If the N64 did what the playstation and saturn did and just have dedicated video/system RAM, and made this RAM relatively low latency SDRAM instead of the relatively high latency RDRAM, this 4kB limitation wouldn't have mattered.
Larger cartridges (i.e. 32/64MByte) gave them space in ROM to play with tiled textures. Usually this -did- also involve use of the 4MB RDRAM upgrade.
#1 It was still slower than the cache.
#2 You were still using the single shared bus. You would still be using cycles which contribute to data stalls elsewhere in the system.
#3 ROM was expensive. N64 games were typically in the ballpark of $10 more expensive than Playstation or Saturn games because of the manufacturing expense.
#4 I don't fully understand why, but it was all or nothing. You couldn't have uncompressed textures in ROM but also gain the benefit of the cache. Maybe the cache invalidation was poor or something. I wish I knew more.
Later games were more likely to go this route because ROM was cheaper. (Moore's Law and all that)
Additionally, the TMEM could only be loaded from RDRAM, not directly from the cartridge. I think the RDP's DMA master is only connected to the RDRAM slave port and not the main system's bus matrix.
So going back to it, games would a lot of the time store compressed data with a simple algorithm that could run out of the CPU's cache. Then the scheme looks like
* Cart->RDRAM DMA of compressed texture
* CPU decompresses texture into another RDRAM bank, and can be considered a RDRAM->RDRAM transfer. Sometimes the RSP handles this instead. I'm not sure if you could load straight out of RSP DMEM to avoid another bounce to RDRAM. I don't think XBUS works that way, but I could be wrong.
* RDRAM->TMEM DMA of uncompressed texture
Interestingly, games with more advanced texturing schemes like Indiana Jones tended to use uncompressed textures. They did this to avoid the decompression step and it's bandwidth. At that point it's just staging the texture with that cart's DMA, and slurping that into TMEM without any other processors eating bandwidth in between.
The Playstation by comparison used EDO (Based on eyeballing the pictures on wikipedia, baseline was 70ns/60ns for CPU and Video memory.) But, It's main bus was under 133mb/sec, and the fastest it could read from CD was 300kb/sec.
EDO Memory would have kneecapped the N64 from a memory bandwidth standpoint. The cartridge bus alone is over 200mb/sec. SDRAM -might- have done the job but may have wound up being more expensive; PC-66 (we are at the infancy of SDR in 1996) would have meant a PCB with 8 chips laid out for the parallel bus. To be frank I'm not sure Nintendo could have even gotten such a configuration (i.e. 8 512KB PC-66 chips.)
RDRAM was definitely a design compromise, but in retrospect I understand it's use in keeping overall costs down.
Dedicated video ram would have been a better option however, but I think it was another cost issue.
The N64 launched at $200, the Voodoo at $300. Of course you would additionally need a computer to run the Voodoo, but I remember thinking the N64 was already way too expensive back in the day. It would've been even more expensive to support a 64-bit memory bus.
[1] There are other factors of course: SD TV resolutions are low so when you hook a 32-bit 90s console up to a large modern flat panel HD or 4K TV with games running at a resolution of around 320 x 240 the pixels are MASSIVE. In addition polygon counts are low, draw distances are often low, and so it goes on. Depending on your setup games can look considerably worse on a modern TV than they would have done on more modestly sized 90s CRT screens. To be clear I'm talking about SD TVs here, not CRT monitors, which could support much higher resolutions and would therefore suffer from some of the same problems as modern flat panels in terms of making the graphics look too sharp.
That being said, going to a real cache rather than a block of manually managed memory I thin would have been the better design choice. Most textures didn't fill up the memory because of the practicalities of managing that memory. A double buffer scheme to load a block wile rendering from another one, the fact that you have to eat the whole cost of the full texture's load before you can render from it, etc.
I bought one on eBay, and it's a quirky device. It's essentially a glorified floppy drive with proprietary disks, and it makes loud, albeit amusing scanning noises.
Every Nintendo console prior to the Wii had some sort of "expansion" or upgrade capability for future peripherals to be added. Hardware revisions took the place of this model.
Plus, adding more texture memory would have meant respinning the RCP silicon. That would have been a significant expense for marginal returns.
Besides, Nintendo had already sold a memory upgrade in the form of the Expansion Pak. A second memory upgrade which couldn't even be installed into existing consoles would have been a very difficult sell, and could have soured customers on their future consoles. ("Why buy a Game Cube when they'll just release a Game Cube Plus next year?")
the https://en.wikipedia.org/wiki/Nintendo_64_accessories#Expans... raised main memory from 4mb to 8mb.
it was required for one of the Zelda games, but also helped with graphics for some games if it was inserted.
seemed to be a decent success.
https://www.reddit.com/r/n64/comments/ft63zn/dk64_memory_lea...
The lead artist claims they’d always been planning to make use of it. There’s compelling evidence of a memory leak or similar issue since the released version of the game apparently crashes if you leave it running for some amount of time over 10 hours (which wasn’t usually a problem in practice except for very long speedruns until it was released on Wii U Virtual Console and people started using save states which don’t reset the timer the same way as turning the game off and on does).
I _think_ it was on the documentary clips shipped with Rare Replay, but I'm not 100% on that.