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 The next design - a octal 20 bit cpu 
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Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Going back to more 1968 style computer. Word addressing. Accumulator design rather a register to register design.
Subroutine calls on even bountry, and single word 19 bit addressing. This way I can use CALL theading for threaded code. 8 registers including
the PC.
More inspection of the new oder code, conficts with some other decoding,thus I am back to two word calls and the orignal order code.


Thu Apr 04, 2024 7:32 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Moving clock generation to the ALU card as well. The ALU gets to use the master clock first.
Moving up to 1984, with a 1.5 uS memory cycle time -- ram . A 1977 version 1.8 uS core or ram.
PS: Looking at the chips I have, I may tweek it for 150 ns ram, and add a wait state for IO.


Thu Apr 11, 2024 6:09 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Gone back to a hex format.
The schematics are roughed out, but no layout is done yet. This will be a emulated design with stubbed IO
routines, for the next few months. Getting close to a "hello world" type test program.
PS: Last minute addition - signed bytes (10 bits).


Sun Apr 28, 2024 5:42 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Changed from Big Endian to Little Endian, so I can read and write disk blocks on the PC and port over to Compact Flash
later. Disk IO would be a modified IBM patter drive from 203 tracks to 225 tracks. 4Kb per track like the PDP-8, 10 bit bytes, GCR. Some sort of strange fat system.
Now I need a name for this beast, and time for software.


Mon Apr 29, 2024 5:55 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
I have the emulator printing "Hello world".
I got dosbox-x running and cross compiling small c version 1.2.
I need to write a simple FAT file system ( 8 inch floppy 10 bits/byte GCR single sided)
and the small C libs. System setup 462 Kb per floppy, two floppies per system. 48K ram + ROM BIOS
+ 2 serial at 1200 baud?
about 1976.


Sat May 11, 2024 8:47 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
I have improved the design, since I could only find 18 bits, in the bit bucket.
This is a serial design, 3 bit nibble. The only rare parts are a 13 mhz osc,
CY7C122 (256 x 4 ) 25 ns ram and a 74H74.

2.5 uS memory cycle time.
Operate 2.5 uS
auto 5.0 uS
index 7.5 uS
The emulator will print "hello world", but more testing needs to be done.
Ben.


Sun May 19, 2024 10:32 pm

Joined: Fri May 24, 2024 1:46 pm
Posts: 1
oldben wrote:
3 bit nibble

I've never heard of a n[yi]bble being anything other than 4 bits.


Fri May 24, 2024 2:05 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
256 wrote:
oldben wrote:
3 bit nibble

I've never heard of a n[yi]bble being anything other than 4 bits.

Next you will tell me a byte is 8 bits.
Other notes.
The design of the serial 18 bit cpu is on hold, just too slow, and floating point
is still too small.
The order code of my 20 cpu will be revised from what I learned on the 18
bit machine, with 31,1,8 float.
Ben.
PS: Evil laugh, bytes are now 10 bits, and nibbles half that.


Sat May 25, 2024 5:17 pm

Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1789
(Welcome, 256! It does seem unconventional for a nibble not to be 4 bits, but as Ben notes, sometimes a byte isn't 8 bits, certainly on this forum where any CPU architecture decision is open for experimentation...)


Mon May 27, 2024 5:36 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Code:
draft #1

 The ben 20 is a medium speed digital computer using LS TTL parts with a
a word width of 20 bits and a byte with of 10 bits. 9 digit binary floating
point.4K dynamic ram or 1.8 uS core memory provide the base 48Kb of memory
with 32kb ram and ROM/CORE for bootstrap and other operating system
routines.
Emulation of the CPU, will use later PAL and bitslice parts.

Cpu model:

The cpu has a Carry flag and 4 general purpose registers A,B,C,D and 3
index registers S X Y and instruction pointer Z (zero).
Index registers do not affect the carry.

Adressing modes are register indirect:
            +r -r r% r+ r-
indexed:    r n
immediate:  # n

opcode postfix  .   byte
                %   register

Instruction format:

  modes:
  0     nop and shift

        nop
     5    4    3    2    1   
  +----+----+----+----+----+
  |0000| AAA|  0 |    :    |
  +----+----+----+----+----+

        shift ac

     5    4    3    2    1   
  +----+----+----+----+----+
  |SSSS| AAA|  0 |    :    | NORMAL
  +----+----+----+----+----+

  1     set condition
        if ac condtion is true set ac to constant
        else set false (0)

     5    4    3    2    1   
  +----+----+----+----+----+
  |NCCC| AAA|  1 |    :+###|
  +----+----+----+----+----+

 2     memory ref reg indirect
    (EFA)= AAA  (STORE)
    AAA  = AAA OP (EFA)
 

     5    4    3    2    1   
  +----+----+----+----+----+
  |COOO|PAAA|  2 |BXXX:+###|
  +----+----+----+----+----+
    P PRE ADDITION
    B BYTE

 3     memory ref INDEXED
    (EFA)= AAA  (STORE)
    AAA  = AAA OP (EFA)
 
     5    4    3    2    1   
  +----+----+----+----+----+
  |COOO| AAA|  3 |BXXX|    |
  +----+----+----+----+----+
  |####|####|####|####|####|
  +----+----+----+----+----+
   
 4    jump to subroutine
      memory ref BYTE imediate

      JSR register indirect
      T = (efa)
      (--S)= Z,Z=T
 
     5    4    3    2    1   
  +----+----+----+----+----+
  |0000|PAAA|  4 | XXX|+###|
  +----+----+----+----+----+
    P PRE ADDITION

    memory ref BYTE imediate
    AAA = AAA OP #
     5    4    3    2    1   
  +----+----+----+----+----+
  |COOO| AAA|  4 |+###|####|
  +----+----+----+----+----+
   
   
 5   lea and control
     lea
 
     5    4    3    2    1   
  +----+----+----+----+----+
  |0000| AAA|  5 | XXX|    |
  +----+----+----+----+----+
  |####|####|####|####|####|
  +----+----+----+----+----+
    control
     5    4    3    2    1   
  +----+----+----+----+----+
  |ffff|    |  5 |    |    |
  +----+----+----+----+----+

     5    4    3    2    1   
  +----+----+----+----+----+
  |0001|    |  5 |    |    |  HALT
  +----+----+----+----+----+

 6   jump on condition
     register indirect
 
     5    4    3    2    1   
  +----+----+----+----+----+
  |NCCC|PAAA|  6 | XXX|+###|
  +----+----+----+----+----+

     
 7   jump and register operate
     jmp indexed
 
     5    4    3    2    1   
  +----+----+----+----+----+
  |0000| AAA|  7 | XXX|    |
  +----+----+----+----+----+
  |####|####|####|####|####|
  +----+----+----+----+----+
   
     reg operate
     AAA = AAA OP XXX

     5    4    3    2    1   
  +----+----+----+----+----+
  |COOO| AAA|  7 | XXX|    |
  +----+----+----+----+----+
 
OPCODE TABLES:
     REGISTER

          AAA   XXX       
     0    A     A         
     1    B     B
     2    C     C
     3    D     D
     4    S     S
     5    X     X
     6    Y     Y
     7    Z(PC) Z(ZERO) OR #

     MEMORY REF

     OOO  OP       
     0    ST    (EFA) = AAA           
     1    ADD/C  AAA  = AAA +  (EFA) + ?0,C
     2    SUB/C  AAA  = AAA + ~(EFA) + ?1,C
     3    CAD/C  AAA  = ~AAA + (EFA) + ?1,C
     4    LD     AAA  = (EFA)
     5    OR     AAA  = AAA |  (EFA)
     6    AND    AAA  = AAA &  (EFA)
     7    XOR    AAA  = AAA ^  (EFA)

    SHIFT
    LATER
   
    LOGIC CC
    LATER





Wed May 29, 2024 12:46 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
I have made a minor change in the order code, moving the B flag around.
I have the 8080 2.1 version of the Small C Compiler generating code to self compile.
Not running yet, but compiles to $09C00 Bytes. No library functions included yet.
Base memory 64kb.


Wed Jun 05, 2024 5:12 am

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
More bug fixes.
Hello world compiles and prints.


Sat Jun 08, 2024 1:43 am

Joined: Sat Feb 02, 2013 9:40 am
Posts: 2104
Location: Canada
How far is it off using an 80x86 compiler? If it had the four segment registers it might be easier to program. Seg. registers are fairly resource frugal.

_________________
Robert Finch http://www.finitron.ca


Sat Jun 08, 2024 8:44 am WWW

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
The whole point of 20 bits, is no segments.
I am sticking to 8080 code model for small c, because the code is a mess for the x86.
I need to check now for alignment issues, as my cpu needs word aligned data, the 8080
does not. I also need to turn off constant folding.


Sat Jun 08, 2024 3:37 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
Revised the order code to make better use of the 2901 bit slice
and have 13 registers and S,Z and Pc registers. 1.8 us Core memory
Cycle time.
The 8080 version self compiles now to "Me Too 75" order codes.
Partial conditional support with side effects and sign extend for long data.
AC(LOW) := AC(LOW) or AC(HIGH) is the side effect after a JUMP on condition.
SET on condition only affects AC(LOW).


Thu Jun 13, 2024 2:39 am
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