Reading a bit from memory

Question

I'm looking into reading single bits from memory (RAM, harddisk). My understanding was, one can not read less than a byte. However I read someone telling it can be done with assembly. I wan't the bandwidth usage to be as low as possible and the to be retrieved data is not sequential, so I can not read a byte and convert it to 8 bits.

Answer 1

I don't think the CPU will read less than the size of a cache line from RAM (64 bytes on recent Intel chips). From disk, the minimum is typically 4 kiB.

Reading a single bit at a time is neither possible nor necessary, since the data bus is much wider than that.

Answer 2

It used to be, say 386/486 days, where a memory was a bit wide, 1 meg by 1 bit, but you will have 8 or some multiple number of chips, one for each bit lane on the bus, and you could only read in widths of the bus. today the memories are a byte wide and you can only read in units of 32 or 64 or multiples of those. Even when you read a byte, most designs fill in the whole byte. it adds unnecessarily complication/cost, to isolate the bus all the way to the memory, a byte read looks to most of the system as a 32 or 64 bit read, as it approaches the edge of the processor (sometimes physical pins, sometimes the edge of the core inside the chip) is when the individual byte lane is separated out and the other bits are discarded. Having the cache on changes the smallest divisible read size from the memory, you will see a burst or block of reads.

it is possible to design a memory system that is 8 bits wide and read 8 bits at a time, but why would you? unless it is an 8 bit processor which you probably couldnt take advantage of a 8bit by 2 gig memory. dram is pretty slow anyway, something like 133 mhz (even your 1600mhz memory is only short burst as you read from slow parts, memory has not gotten faster in over 10 years).

Hard disks are similar but different, I think sectors are the smallest divisible unit, you have to read or write in those units. so when reading you have a memory cycle on the processor, no different that going to a memory, and depending on the controller either before you do the read or as a result, a sector is read of the disk, into a buffer, not unlike a cache line read, then your memory cycle to the buffer in the disk controller either causes a bus width read and the processor divides it up or if the bus adds complexity to isolate byte lanes then you isolate a byte, but nobody isolates bit lanes. (I say the word nobody and someone will come back with an exception...)

most of this is well documented, not hard to find. For arm platforms look for the amba and/or axi specifications, freely downloaded. the number of bridges, pcie controllers, disk controller documents are all available for PCs and other platforms. it still boils down to an address and data bus or one goesouta and one goesinta data bus and some control signals that indicate the access type. some busses have byte lane enables, which is generally for a write not a read. If I want to write only a byte to a dram in a modern 64 bit system, I DO have to tell everyone almost all the way out to the dram what I want to write. To write a byte on a memory module which must be accessed 64 bits at a time, at a minimum a 64 bit read happens into a temporary place either the cache or the memory controller, then the byte to be written modifies the specific byte within the 64 bit word, then that 64 bit quantity, eventually, is written back to the memory module itself. You can do this using a combination of the address bits and a few control signals or you can just put 8 byte lane enables and the lower address bits can be ignored. Hard disk, same deal, have to read a sector, modify one byte, then eventually write the whole sector at a time. with flash and eeprom, you can only write zeros (from the programmers perspective), you erase to ones (from the programmers perspective, is actually a zero in the logic, there is an inversion) and a write has to be a sector at a time, sectors can be 64 bytes, 128 bytes, 256 bytes typically.

Answer 3

You cannot read less than a byte from any PC or hard disk that I know of. Even if you could, it would be extremely inefficient.

Some machines do memory mapped port io that can read/write less than a byte to the port, but it still shows up when you get it as at least a byte.

Use the bitwise operators to pick off specific bits as in:

char someByte = 0x3D;  // In binary, 111101
bool flag = someByte & 1; // Get the first bit, 1
     flag = someByte & 2; // Get the second bit, 0
// And so on.  The number after the & operator is a power of 2 if you want to isolate one bit.
// You can also pick off several bits like so:
int value = someByte & 3;  // Assume the lower 2 bits are interesting for some reason