I am learning computer organization but struggling with the following concept. In non-DMA scenarios, do all disk reads follow the following sequence to get into main memory:
Disk storage surface -> Disk registers -> CPU registers -> Main memory
Similarly for writes, is the sequence:
Main memory -> CPU registers -> Disk registers -> Disk storage surface
(I know that in a DMA scenario, the CPU only initiates the transfer after which the content of the disks are transferred directly to main memory).
If yes, before DMA came, was the above sequence a serious bottleneck as overall CPU registers' capacity is much less compared to main memory and storage disk? Or it is so fast that a human user won't notice in non-DMA modes?
PS: Please bear with my rudimentary terminology, but I hope I conveyed what I want to ask.
Yes, what you describe is what happened in the bad old days with programmed-I/O instead of DMA.
For example, IDE disk-controller hardware used to be less well standardized, so the Linux drivers defaulted to programmed I/O (i.e. a copy loop using x86 IN instructions, since ATA predated memory-mapped I/O registers being common). For decent performance, you had to manually enable DMA in your boot scripts.
But before doing that, check by manually enabling DMA it didn't lead to lockups, or far worse cause data corruption.
re: memory-mapped file: nothing to do with how the data gets from disk into the pagecache (or vice versa). mmap() just means your process's address space includes a shared mapping of the same pages that the OS is using to cache the file's contents.