No, that’s not what RISC is about. There was some early attempts to keep the number of instructions low–originally, ARM didn’t have a multiply instruction, and there’s still a bunch of microcontrollers you can buy that don’t have a divide instruction–but it was quickly abandoned as it’s just not that useful. It only holds back instructions that optimize common cases. Your compiler can implement multiplication by doing addition in a loop, but that’s not very efficient.
What really worked about it was keeping a separation between how memory is accessed. You don’t have an ADD instruction that can fetch from both registers or main memory. You have a MOV instruction that can fetch from memory into a register, and you have an ADD instruction that can work on registers.
I’m a computer engineering major (still a student tbf), I’m well aware of the difference between CISC and RISC, I was making a joke.
Also, I understand your point, but you should know though that a load-store architecture and a RISC instruction set are not the same thing. The vast majority of RISC ISAs are load-store, but not all load-store architectures are RISC.
MOST RISCs:
3a) Have 1 size of instruction in an instruction stream
3b) And that size is 4 bytes
3c) Have a handful (1-4) addressing modes) (* it is VERY
hard to count these things; will discuss later).
3d) Have NO indirect addressing in any form (i.e., where you need
one memory access to get the address of another operand in memory)
4a) Have NO operations that combine load/store with arithmetic,
i.e., like add from memory, or add to memory.
(note: this means especially avoiding operations that use the
value of a load as input to an ALU operation, especially when
that operation can cause an exception. Loads/stores with
address modification can often be OK as they don’t have some of
the bad effects)
4b) Have no more than 1 memory-addressed operand per instruction
5a) Do NOT support arbitrary alignment of data for loads/stores
5b) Use an MMU for a data address no more than once per instruction
6a) Have >=5 bits per integer register specifier
6b) Have >= 4 bits per FP register specifier
Note that none of this has to do with reducing the number of instructions, which is what people tend to think of when they hear the name.
All instructions occupy the same amount of space in memory.
Both ARM and RISC-V have compressed instructions. Dunno how ARM works but with RISC-V the 16-bit instruction set is freely interspersable with the 32 bit one, which also get their alignment reduced to 16 bits. Gets like 95% of the space reduction possible with full variable-width instructions without overcomplicating the insn decoder.
As to addressing and loads and arithmetic: No such instructions, but every CPU but the tiniest ones are expected to do macro-op fusion for things like indexed loads. Here’s an overview.
The MMU thing… well the vector extension can do gather/scatter, I guess it could stay within the letter of “use the MMU once” but definitely not the spirit.
At this point ARM is a CISC architecture
No, that’s not what RISC is about. There was some early attempts to keep the number of instructions low–originally, ARM didn’t have a multiply instruction, and there’s still a bunch of microcontrollers you can buy that don’t have a divide instruction–but it was quickly abandoned as it’s just not that useful. It only holds back instructions that optimize common cases. Your compiler can implement multiplication by doing addition in a loop, but that’s not very efficient.
What really worked about it was keeping a separation between how memory is accessed. You don’t have an ADD instruction that can fetch from both registers or main memory. You have a MOV instruction that can fetch from memory into a register, and you have an ADD instruction that can work on registers.
ARM still does this just fine.
I’m a computer engineering major (still a student tbf), I’m well aware of the difference between CISC and RISC, I was making a joke.
Also, I understand your point, but you should know though that a load-store architecture and a RISC instruction set are not the same thing. The vast majority of RISC ISAs are load-store, but not all load-store architectures are RISC.
http://www.quadibloc.com/arch/sriscint.htm
https://groups.google.com/g/comp.arch/c/IZP5KUJprHw?pli=1
Note that none of this has to do with reducing the number of instructions, which is what people tend to think of when they hear the name.
Both ARM and RISC-V have compressed instructions. Dunno how ARM works but with RISC-V the 16-bit instruction set is freely interspersable with the 32 bit one, which also get their alignment reduced to 16 bits. Gets like 95% of the space reduction possible with full variable-width instructions without overcomplicating the insn decoder.
As to addressing and loads and arithmetic: No such instructions, but every CPU but the tiniest ones are expected to do macro-op fusion for things like indexed loads. Here’s an overview.
The MMU thing… well the vector extension can do gather/scatter, I guess it could stay within the letter of “use the MMU once” but definitely not the spirit.
Someone confusing load-store with RISC again.