XPost: comp.sys.ibm.pc.classic, comp.sys.ti, comp.sys.sinclair   
   From: ReapNewsB@bullet3.fsnet.oc.ku   
      
   A936@hotmail.com wrote:   
      
   ...   
      
   > As mentioned, port i/o mappings are totally dependent on the hardware   
   > and the designers.  One particularly important distinction between   
   > the z80 and the 8080 is that on the former i/o ports are 16-bit whereas   
   > on the latter they are only 8-bit.  On the z80, it is not uncommon to   
   > continue to treat its i/o space as 8-bit or to treat the lower 8-bits   
   > as   
   > identifying a specific device and the top 8-bits to select registers on   
   > the device.  The z80's instruction set also "prefers" 8-bit i/o ports.   
      
   AFICR, the Z80 when using the C reg to define which port to use (on the   
   lower 8 address lines, as in IN A,(C) or OUT A,(C) ) also puts out the B reg   
   on the top 8 address lines - the zx81 uses this to scan the keyboard with   
   the B reg activating the "current row".   
      
   When the immediate mode version is used (as in IN A,80 or OUT A,80) I'm not   
   sure, but somewhere lurking round my memory suggests, that the current   
   contents of the A reg is used for the top 8 bits - can anyone confirm/deny   
   this?   
      
   None of my (rather scant) Z80 programming books describe the ports as   
   "16-bit" addressed, only 8-bit with the port number either as immediate data   
   or in the C reg.   
      
      
   >   
   > It might be interesting to see if early z80 basics that supported   
   > port i/o also supported 8-bit i/o only.   
   >   
   >   
      
      
      
   --   
   Outgoing mail certified virus free...   
   It has gone nowhere near a Windwos machine before my ISP.   
   I am the "ILOVEGNU" signature virus. Just copy me to your   
   signature.  This email was infected under the terms of the GNU   
   General Public Licence.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)