Last visit was: Wed May 29, 2024 6:47 pm
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It is currently Wed May 29, 2024 6:47 pm
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The next design - a octal 20 bit cpu
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Thu Apr 04, 2024 7:32 am |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Thu Apr 11, 2024 6:09 am |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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).
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| Sun Apr 28, 2024 5:42 am |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Mon Apr 29, 2024 5:55 am |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Sat May 11, 2024 8:47 pm |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Sun May 19, 2024 10:32 pm |
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256
Joined: Fri May 24, 2024 1:46 pm
Posts: 1
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oldben wrote: 3 bit nibble I've never heard of a n[yi]bble being anything other than 4 bits.
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| Fri May 24, 2024 2:05 pm |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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.
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| Sat May 25, 2024 5:17 pm |
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BigEd
Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1787
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(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...)
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| Mon May 27, 2024 5:36 am |
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oldben
Joined: Mon Oct 07, 2019 2:41 am
Posts: 601
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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
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| Wed May 29, 2024 12:46 am |
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