[Coco] 6809 CPU architecture

[jimcox at miba51.com]

Roger:

Thanks for the pointers, keep them coming :)

Jim 

On Tue, 20 Jan 2004 00:34:53 -0500
  Robert Gault <robert.gault at worldnet.att.net> wrote:
>
>6809 Registers
>
>bit #	15                                    0
>         ---------------------------------------
>                   X - Index Register
>         ---------------------------------------
>                   Y - Index Register
>         ---------------------------------------
>                 U - User Stack Pointer
>         ---------------------------------------
>               S - Hardware Stack Pointer
>         ---------------------------------------
>                  PC - Program Counter
>         --------------------|------------------
>            A - Accumulator  | B - Accumulator
>         ---------------------------------------
>         \___________________D_________________/
>
>                             -------------------
>                              DP - Direct Page      MSB 
>of 16 bit address
>                             -------------------
>                              E|F|H|I|N|Z|V|C        CC - 
>Condition Code
>                             -------------------
>
>E = Entire Flag
>F = Fast Interrupt Mask
>H = Half Carry
>I = Interrupt Mask
>N = Negative Flag
>Z = Zero Flag
>V = Overflow Flag
>C = Carry Flag
>
>Registers are multi-purpose. Math usually is done with 
>the A, B, or, A+B ie. D registers but also can done with 
>the X & Y index registers. Stack registers can be used 
>for storage or transfers as well as the usual stack.
>
>A stack is just like a pile of plates where each plate is 
>a byte of memory. The bottom of the pile is high memory, 
>the top low for the 6809 systems. Typical usage is first 
>in, last out, meaning that with stack operations the top 
>of the pile is what is accessible; ie. push/pull 
>operations.
>
>To do anything with a computer requires a transfer of 
>information through these or similar registers. The flow 
>of information can be to/from hardware or memory.
>
>In addition to data transfers, the 6809 is capable of 
>very simple math; addition, subtraction, 8 bit 
>multiplication. It can also make simple comparisons; =, 
><>, >, <, >=, and <=.
>
>At the bit level, it is possible to rotate the A & B 
>registers so that they act like conveyor belts where data 
>moves either low to high or high to low within the 
>register.
>
>While it may not seem possible to accomplish much with 
>such simple operations, they are the basis for everything 
>the Coco can do which is quite allot.
>
>
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