From: david.brown@hesbynett.no   
      
   On 18/12/2025 23:30, BGB wrote:   
   > On 12/18/2025 12:20 PM, Rosario19 wrote:   
   >> 8 bit cpu for access memory other than 0..255 location has need at   
   >> last one 16 bit register and 16 bits operations, so i think that even   
   >> a 8 bit cpu has to have int in C language as 16 bits   
   >   
   > There are no "true" 8 bit systems in this sense, as pretty much every   
   > existing 8-bit CPU has had support for 16-bit operations in some way,   
   > though often by using register pairs.   
      
   That is, at best, an exaggerated interpretation.   
      
   >   
   > So, in this sense, it makes more sense to call them 8/16 instead, as the   
   > exact line between 8-bit machines and 16-bit machines is often a little   
   > fuzzy (and both tend to look basically similar as far as C is concerned;   
   > for the machines big enough to where running C on them is viable).   
   >   
   >   
   >   
   > One could then ask, what would a minimal 8-bitter look like.   
   >    Might make sense to go with fixed-length 16-bit instructions.   
   >    Likely 8x8b registers;   
   >    ...   
   >   
   > Say:   
   >    AL, AH   
   >    BL, BH   
   >    CL, CH   
   >    SP, LR   
   >   
   > If you want a 16-bit ADD, you do two:   
   >    ADD, ADC   
   >   
      
   There are lots of 8-bit processors with /far/ fewer registers than that.   
      
   The 6502, had :   
      
   	A = 8-bit accumulator   
   	X = 8-bit index register   
   	Y = 8-bit index register   
      
   	S = 8-bit stack pointer   
   	P = 8-bit flag register   
   	PC = 16-bit program counter   
      
   Access to the zero page was fast - access to memory beyond that was done   
   (IIRC) with a 16-bit base address stored in the zero page memory, added   
   to an 8-bit index register.  There were no 16-bit registers, no register   
   pairs (though I think some 6502 variants could use an XY pair for   
   addressing), no 16-bit operations.   
      
   A microcontroller I used on a lot of projects was the COP8.  It had one   
   8-bit register "A", and a 16-bit PC.  Access to wider memory was via paging.   
      
      
   Devices with only 8-bit program counters were typically only for very   
   niche uses where cost was absolutely critical - there's not a lot you   
   can do with 256 bytes (or perhaps 256 16-bit words) of program space.   
   But there were plenty of 8-bit microcontrollers where the PC was not   
   really 16-bit, but was more of an 8-bit "PC_lo" register combined with a   
   paging register (which might have been a fair bit smaller than 8-bit).   
   Code flow would wrap within a page rather than moving seamlessly on to   
   the next page.  (Of course all such systems were Harvard architectures   
   with completely separate code and data.)   
      
      
   And at one time, 4-bit microcontrollers were the dominant architecture   
   in terms of the number of devices made and sold.  These usually had a   
   way to combine two registers to get 8-bit address registers.   
      
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