Maybe they only recently decided to target RISC-V but the ALU design does not cover all the functions needed. It is missing implementation of the shift instructions, for that they need a 32 bit 5-level barrel shifter. Plus they also need a comparison block which is quite complex on its own. Given the number of years it has taken to get to this point, if could be 2050 before it is working.
You don't need a barrel shifter. You can shift one bit at a time. It's slower, but takes far fewer components.
Capacitor memory addressed with tubes seems a strange choice. DRAM is capacitor memory, of course. Atanasoff, who had a a sort of computer in 1939, had capacitor memory, but he had to use a drum rotary switch to address it.
Memory was the big problem in the early days. IBM had an electronic multiplier before WWII, and plugboard-programmed machines, but no good memory elements. (Just registers with motor driven wheels and clutches and contacts.) Pilot ACE had a delay line (slow, serial access), the Manchester Baby had a Williams tube (too expensive per bit, but random access), and the EDVAC had mercury tank delay lines (slow, serial access, and toxic). Whirlwind (1951) had the first core memory (expensive per bit, but got cheaper over time.)
Core would be a reasonable choice for a tube system. Addressing is XY, so you need O(sqrt(N)) tubes.
Memory was a million dollars a megabyte as late as 1970.
I agree. Most people don't realize just how big a problem memory was in the early years of computers. People think about how to implement the ALU with various technologies, but memory is where things get sticky.
Core memory was much better than the earlier approaches you described, and is really the only way to go for a pre-DRAM computer. Once core memory came along, the other approaches vanished. The IBM tube computers used dynamically-refreshed capacitors for register storage (called a Havens Delay Unit), but I agree that capacitor memory is a strange choice for main memory since it needs O(N) tubes.
> You don't need a barrel shifter. You can shift one bit at a time. It's slower, but takes far fewer components.
I think I've finally found someone to ask this question.
How many transistors and space did it take to implement a 8-bit barrel shifter in the 1980s? I found the computing capabilities of 8-bit CPUs in retrocomputers, like the Z80 or 6809, were actually not too bad. With shift-and-add, a lot of computation can be done effectively, but their biggest limitation is the lack of a barrel shifter. Without one, no constant-time bitshift is possible with variable step, you have to shift 1-bit at a time. Performance could be boosted dramatically if a single 8-bit barrel shifter is included, and would open the avenue of a lot of optimization techniques for graphics as well.
Why didn't they include one? Especially when you consider that the Z80 was an "ultimate upgrade" of the Intel 8080, and the 6809 was the "ultimate upgrade" of the 6800.
Was it technical limitations, i.e. even a 8-bit barrel shifter is still too expensive for a 8-bit chip? Or was it that the cost of additional shift instructions decoding/processing is much higher than the barrel shifter itself? Or was it the lack of demand, i.e. it could be done reasonably well with acceptable cost, but simply no commercial reason in the 8-bit era to add one?
I examined the ARM-1's barrel shifter on the die [1]. It takes roughly 10% of the chip so it's a fairly hefty investment.
8-bit chips probably didn't have a barrel shifter because they were very limited in functionality. (I didn't realize how limited until I looked at mainframes of the time.) Functions like multiply and divide are more useful than barrel shifting, and those were missing too. Barrel shifting is sort of a frill. As for the Z80, it was very, very tight on space so it's not surprising the barrel shifter was lacking. It didn't even have an 8-bit ALU; it had a 4-bit ALU that was used twice per operation.
> I examined the ARM-1's barrel shifter on the die. It takes roughly 10% of the chip so it's a fairly hefty investment.
> Functions like multiply and divide are more useful than barrel shifting, and those were missing too.
My thought was that the lack of multiplier/divider can be somewhat compensated by adding a cheaper 8-bit barrel shifter instead. So according to your analysis, apparently even that didn't make economic sense for a 8-bit chip.
Thanks a lot for giving an authoritative assessment, Ken!
Most of the 8-bit microprocessors I'm aware of -- including the Z80 and 6800 -- only had single-bit shift/rotate instructions, which didn't require a barrel shifter. Variable shift instructions were comparatively rare.
Besides, a 32-bit barrel shifter is considerably more complex than an 8-bit one. It's not just four times larger to deal with the wider operands; it also needs another two levels of muxes, making it closer to 6x as complex.
> most of the 8-bit microprocessors only had single-bit shift/rotate instructions
Yes, they don't have a barrel shifter because they don't have variable shift instructions. But unfortunately, you didn't answer the actual question: Why didn't they have it then? As I've already asked, Was it technical/cost limitations? Or was it simply the lack of demand/commercial reasons to add one?
