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CPU and registers

Posted on 1998-11-20
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Last Modified: 2010-04-27
I am a student informatics and my teacher asked uw to look up the following thing:
A (8086) CPU has 7 registers ..
IR  PC  SP  R0  R1  R2  R3      
The question was why they start with R1 afther SP in stead of staring
with R1 and ending with R4   ... if you know the answer you would help me
alot by explaining it to me ;-)

Sk
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Question by:wcmwcm
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by:mark2150
ID: 1129423
That is *NOT* the internal model of the 8086 CPU!

AX, BX, CX, DX, SI, DI, IP, CS, DS, ES, SS, SP, flags

If you want to see the registers first hand get to DOS prompt and type:

DEBUG
R
Q

This will display registers for you.

-r
AX=0000  BX=0000  CX=0000  DX=0000  SP=FFEE  BP=0000  SI=0000  DI=0000  
DS=176E  ES=176E  SS=176E  CS=176E  IP=0100   NV UP EI PL NZ NA PO NC
176E:0100 7415          JZ      0117                              
-q


AX, BX General Purpose Registers
CX       Counter Register/General Purpose register
DX       I/O Pointer Register/General Purpose register
SI/DI    Source and Destination Index registers for string moves and data arrays
CS:IP   Code Segment/Instruction Pointer - what's executing
DS       Data Segment, implied segment for most data ops
ES       Extra Segment, Implied segment register for DI
SS:SP  Stack Segment - stack pointer is here
BP       Base Pointer, alternate stack
Flags   Condition Code register. Internal processor state flags, zero, neg, etc.

Have no clue what CPU that your register model is with. It's unfamiliar and certainly NOT 8086! Have written ASM in 8080A, Z80, 80x86, 6502, 6800, 68000, SEL 32/55, Z8000, & DEC 11-780 and none of *them* match what your teacher says the model is.

M


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LVL 12

Accepted Solution

by:
mark2150 earned 100 total points
ID: 1129424
Thinking more aboutyour question... If you want to know why the register numbers are zero based (R0-R3) instead of one based (R1-R4) is because all zeros is simplest bit pattern. Internally to CPU chip four registers are selected with two internal bit lines, Possible values are:

0 0 - 0
0 1 - 1
1 0 - 2
1 1 - 3

Register numbers therefore correspond to internal electrical states.

M

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