68k homebuilts

[bfranchuk at jetnet.ab.ca]

Chuck Guzis wrote:
> On 12 Dec 2008 at 10:47, bfranchuk at jetnet.ab.ca wrote:
>
> How about a 20-bit word size, 4 bit opcode, 65KW addressing space, 
> simple one-address+accumulator machine?  Enough directly-addressable 
> memory to do just about whatever you need to do, simple to implement.
>
> Cheers,
> Chuck
>
>   
That was last weeks design. :(
That is 3 opcode bits  and a indirect bit. Going to 4 opcode bits
gives you 32K of memory and would make a nice small machine
that dates around the early 60's.

The problem is that I want to have local function variables, and that 
requires
a stack frame pointer and one index register at least for structure address.
(A 6809 DP style register provides global variables, but hardware 
relocation is
not possible.) This architecture requires a register file and alu design 
rather than
the dedicated logic of a AC,PC,MAR and MBR on a classic machine.
The CPLD's don't have ample hardware for a complex machine like a PDP-11
but they do just have the capacity for machine's hardware somewhere between
the 6800 and the 6809.

Word and byte operations for  LD,ST,ADD,SUB,AND,OR,XOR.  {AC,IX,SP)
Addressing modes
1)  # long   :immedate long
2)  #short   :immedate short
3)  (0,dp,ix,sp) +  nnn  : indexed
4) @ (0,dp,ix,sp)+ nnn : indirect indexed.
Direct and  # addressing modes AC ONLY
JMP,JMS,JZ,JNZ,ADC,SBC
Direct only - INCRIMENT/DECRIMENT MEMORY
INC,IN2,DEC,DE2
Special function - AC,IX OPERANDS
LEA,STX,NEG,NGC,SCC,{SHIFT},EI,DI