z8000 segment scheme question - test-drb@ccmp.vtda.org

26 Jul 2004

The Z8000 segmented address scheme has two forms. The long form that you
described uses two 16 bit values (registers or memory locations). The
short form uses a single 16 bit value with the 7 bit segment number in
bits 9-15 and an 8 bit offset into the segment in bits 1-8. Bit 16 of
the first 16 bit value is a flag which indicates long or short segment
address. This is important for memory addresses operands as the CPU can
get an address word operand from memory, and then based on the flag bit
can decide if the next word must be read to get the complete long
address. In either case the segment address is located in the same place
in either a short segment address or the first word of a long segmented
address.

David

...
  -----Original Message-----
 From: cctalk-bounces(a)classiccmp.org 
 [mailto:cctalk-bounces@classiccmp.org] On Behalf Of SHAUN RIPLEY
 Sent: Monday, 26 July 2004 1:27 PM
 To: cctalk(a)classiccmp.org
 Subject: z8000 segment scheme question

 I picked up one of my computer books today and read
 that z8000 uses one 16 bit register to hold the 7 bit
 segment number and one register to hold the 16 bit
 offset. The strange thing is that the segment number
 is hold in position of bit 9-14 other than the bottom
 half of the first register. I goggled and found
 complaint about this scheme but no one explained why
 it was designed so. Could somebody on the list tell me why?

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