Kobold K2 - RISC TTL Computer

A 16 bit RISC computer with video display, from just a few TTL and memory chips. Online C compiler and simulator available.

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The Kobold K2 CPU will be on a single pcb, constructed from TTL IC's.

Its main characteristics are:
- 16 bit processor, 16 bit databus
- 8 registers
- can access one Megabyte of memory
- no microcode
- every instruction executes in two cycles

To make it a complete computer, the K2 CPU will be connected to a mainboard, that will have:

- memory (RAM and parallel Flash)
- connector for video system, 80 x 25 characters text, graphics, and sound
- onboard mass storage 32MByte
- glue logic
- I/O connectors

Constraints are:
- low number of parts (TTL)
- no off-the-shelf processor or microcontroller
- no programmable parts in the CPU
- no 74181 ALU

For the CPU part, 44 TTL IC's will be used.

The video system has it's own project page:


After having worked several months on the first Kobold CPU , I got the feeling that it was going in the wrong direction. I was working on a Javascript assembler, and got tangled up in the microcode complexity. I also didn't like that so many parts were needed to decode the microcode. So I decided to make a huge change in the design. Here is Kobold K2 !

So what will change ?
  - Microcode is not used any more, instructions will be RISC
  - Four new 16-bit data registers in hardware (now total 8 registers)
  - The 8-bit ALU will change to 16-bit ALU
  - All instructions need two cycles (fetch, execute) 

The Kobold K2 will be faster, and its operation will be easier to explain.
The video system will stay mostly the same.


Finding the balance between low number of parts and high functionality is one of the key aspects of TTL CPU design (at least, for me it is). I want to keep the part count low, but not to the extreme as in #1 Square Inch TTL CPU. The CPU part of the computer should fit on a single PCB.

To keep the control system simple, every instruction should execute in a single cycle. If the ALU was kept 8 bits wide, that would mean 2 instructions for many 16-bit actions (as in the Z80 or 6502), and that would slow down 16-bit operations. Therefore, the ALU is now 16 bit wide. I don't want to use the 74181 ALU, so to keep part count reasonable, the ALU has only a few functions. The small number of functions also simplifies control.

The average performance per clockcycle is expected to be higher than that of a 6502 or Z80 and might come close to the performance of a 68000 in several situations. The performance is mainly due to the RISC strategy, fast access to 4 data registers and 4 address registers, and to having everything 16 bit wide.


The VGA card provides 640x480, 32K colors, full color sprites, and  sound !


To make programming easy, an Online Javascript Compiler/Assembler/Simulator was made. The C code or assembly code for the K2 processor can be made and assembled in your browser. The C code can be written in the included online editor that has syntax highlighting, matching parenthesis indication and several other features. The C compiler is not yet full-featured, that's a project all on its own.

It can also simulate the cpu. If you open the simulator just press Run to start the prime-number-generation demo ! In this window, you can also open the Manual to see which instructions and addressing modes are available. And of course, you can try to program yourself....

A Raspberry Pi can connect to the Kobold computer. On the Pi, you can make the application for the Kobold with the online compiler, then download the result to the Pi and put it in the Flash program memory of the Kobold (with a Python script).


1.  Operation principle

2. Instruction set

3. Addressing modes

4. Instruction sequencing

5. Subroutines

6. Instruction encoding and conditional branching

7. Schematic of the CPU

8. CPU schematic explained

9. PCB impression of the CPU

10. Changing the memory access model

11. More conventional instruction sequencing

12. New instruction set

13. Hello Simulator !

14. Instruction Map

15. PCB's ordered

16. Online Javascript Assembler/Simulator

17. Started soldering

18. Datapath working !

19. Blinking LED !

20. Milestone: Hello World !

21.  Introducing the C compiler

22. Running C at full speed

23. Never too late to change the instruction set


All instructions are 16 bits long. Most MOV instructions can be executed conditionally. 

Most instructions have two operands:

  • An operand according to one of the 7 addressing modes
  • A register operand (one of the 8 registers)

The register operand is in most...

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KMAIN1948 - Kobold Main.pdf

Kobold K2 Mainboard Schematic

Adobe Portable Document Format - 59.73 kB - 11/29/2019 at 20:59


KCPU1948 - Kobold K2 CPU.pdf

Kobold K2 CPU Schematic

Adobe Portable Document Format - 79.85 kB - 11/29/2019 at 20:58


ms-excel - 50.50 kB - 11/22/2019 at 15:31


circ - 937.97 kB - 11/19/2019 at 10:02


hello 20191115.dmp

Hello World binary, to put in the Logisim RAM

dmp - 119.00 bytes - 11/15/2019 at 15:55


  • Never too late to change the instruction set

    roelh06/27/2020 at 20:31 0 comments

    During work on the C compiler it appeared that the instruction set had some problems. Compiling was possible, but sometimes resulted in code that was longer than optimal. (Of course the lack of AND, OR and XOR also can give longer code, but that was accepted from the beginning and I won't change that. Not now :)  ).

    The main problems were:

    • When storing a register to memory, the contents of that register changes
    • Move from one data register to another one is not possible.
    • Instructions with 3 operands exist, but placed strong restrictions on the involved registers
    • Compare instructions don't exist. An ADD is used instead, with unwanted effect that the sum is written to a register.

    Of course the change must be small, because I will also have to change the hardware prototype and I don't want a lot of rewiring. 


    The main change is the introduction of many 3-operand instructions. The second operand is always a register, and when this is register D3, this transforms a ADD in a MOV because D3 is now always zero. A new RISK feature !

    The other problems were also addressed. Of course new restrictions appear: Register D3 can not be used to store a value. The number of Z-page addresses goes from 128 to 64, and the result of a shift instruction always goes to D2.

    The new features:

    • When storing a register to memory, the contents of the register stays the same.
    • All register-to-register moves are possible.
    • Many 3-operand instructions available.
    • A CMP instruction is introduced (it is an ADD instruction, but only writes CY and doesn't change a register.

    Some other new instructions, that were not in the problem list but are quite useful:

    • A memory position can be cleared without having to load a register with zero (The CLR instruction stores the always-zero D3)
    • Many register-to-register instructions can be conditional (executed when CY is 0 or 1).
    • Many MOV and ADD instructions (also 3-operand versions) can be conditional

    The new set needs one extra AND gate. That's an extra IC. Perhaps a few of the other gates can also get a useful function....



    • change the assembler
    • change the manual of the assembler
    • change the simulator
    • change the C compiler
    • change the hardware

  • Running C at full speed ​

    roelh06/02/2020 at 13:26 2 comments

    It's been a while since the last log. I could not spend much time on the project lately.

    Progress was made on the C compiler. It now handles initializers, strings and chars, all needed for the "Hello World" program. On the hardware side, U41 was placed, that is needed for reading chars from the upper half of a word.

    Finally I was now able to get a real C program running on the hardware ! 

    On the hardware I mounted the 25 MHz oscillator, and selected it by mounting R2 and removing R5 (that connected to the 460 kHz RC oscillator). The timing loops for the synchronous character transfers were now 750 instead of 15, to account for the 50 times higher clock frequency.

    The combined Hello-World and prime number program were also running at the intended speed, 25/4 = 6.25 MHz cpu clock, that is 3.125 million instructions per second (one instruction is a fetch cycle and an execute cycle).

    The simulator runs quite slow, and would be practically useless with two 750 cycle loops after writing a character. The simulator already checks for a write to 0xF000. After writing a character, the simulator has extra code that will directly return from the putchar function, bypassing the loops. Now a single program will run on the simulator as well as on the real hardware.

    So what is next ?

    - start designing the video display unit

    - arrange a keyboard for the Kobold

    - have a small operating system, that itself is programmed in C (suggestion anyone ?)

  • Introducing the C compiler

    roelh03/13/2020 at 21:00 0 comments

    The Kobold K2 processor is quite suitable to run a language like C. 

    Fortunately I had started a compiler project several years ago, but I never finished it. So now I had the opportunity to get some results from all the hours that already went into that project. 

    Another possibility would be to use an existing compiler and change the code generation part. Several C compilers generate pseudo-instructions, that only have to be translated to instructions for your own CPU. But since the Kobold is such an odd beast, it is needed to have control over the code generation at a high level, to get an acceptable code output. And I had the unfinished project as a start point, so that was the way to go.

    My compiler project started in the period that I discovered that you could program a HTML server to generate webpages using the PHP language, and that PHP was actually a full featured programming language. To make a long story short, the compiler was programmed in PHP and ran on the webserver. 

    It generated some intermediate code. Many of my fellow HAD'ers could face a similar simulation (wanting to generate code for your homebuilt processor, but having difficulty in interfacing to an existing front-end compiler). How much easier would it be, if the intermediate code was standardized ? 

    I here propose a standard interface between the parser and code generator. If this interface is standardized, a codegenerator for a certain CPU can be used with all C compilers that comply to this standard. It is described HERE (but is not fully complete yet). Its syntax is JSON.

    In the last months, I added (to my PHP compiler) a section that translated the old intermediate code to the new JSON standard. I then started with the code generation section, programmed in Javascript. The code generator does several optimizations. The resulting assembly code is reasonable compact, and also fast because native code is generated. Finally, the K2 assembler produces the binary code.

    The C compiler is now integrated into the Kobold K2 online Javascript assembler/simulator. The generated assembler code is displayed side-by-side to the C source code. The C code can be written in the included online editor that has syntax highlighting, matching parenthesis indication and several other features (it is the ACE editor that was integrated here). The C compiler is not yet full-featured. Its capabilities are in the Language Manual (also reachable from within the online editor)

    On a single browser page you can now edit, compile, assemble and simulate the C code, and download a file to flash it into the Kobold. With the 'assembler' checkbox, it is still possible to program directly in assembler.

    See it live at !

    The page loads a prime-number generator as demo program. You only have to press the RUN button to start it. However, the simulator is quite slow because it simulates almost at the gate level (see comments in the C code of the demo).

    On the hardware side, the bug in the NOR instruction was found (a short on the pcb), and the RAM is now also working.

  • Milestone: Hello world !

    roelh01/28/2020 at 19:40 2 comments

    A milestone for every CPU project: print "Hello World !"  !


    Having text output could have been a project all on it's own...  but here I use the Raspberry Pi as terminal. On the main board of the Kobold, the connector to the RPi has two pins that are used here:

    • signal SPI_CS3/ pin 11, RPi GPIO 11, clock signal coming from Kobold
    • signal SPI_MOSI / RPI_DATA pin 2, RPi GPIO 13, bidirectional data

    A simple synchronous protocol is used. The Kobold sends a clock signal, and on the data line it puts the character, complete with a startbit (low) and a few stopbits (high). It is intended that the RPi will be able to send data back over the same line, using the other edge of the clock signal. So the RPi can be the terminal for Rx and Tx, as long as there is no other way to do this.

    The Python script at the RPi side got an extra section, started with the 'X' command, to receive the characters and put them on screen.


    A remark about the filename, you see on the screenshot that the TX program is named tx_ca28s41i. The reason is, that when you download the file from the webpage, you will get 3 files:

    • TXT file with source code
    • HEX file to put in the Flash
    • LST file that shows the assembled source code

    During debugging, you will download a lot of times. By always downloading the source at the same time, with the same timestamp (ca28s41i), you can always trace the source that belongs to an executable. And when replacing the binary file in the Python script, you only have to type the four last characters (thats the time-of-day) to replace the binary file with a newer one.


    And here is the program, dominated by the section for serial transmit:

    ; Kobold K2 
    ; program name: TX
    ; character TX to RPi
    ; 20200128
     movp 0,wp ; set wp page to zero
     movp 0x20,a2 ; A2 output address:
    ; A2+16 spi_cs3 low (char clk)
    ; A2+18 spi_cs3 high (char clk)
    ; A2+24 MOSI low
    ; A2+26 MOSI high
    ; A2+28 led on
    ; A2+30 led off
     mov data,A3
     jmp there
     mov D3,A3 ; restore A3
     add 2,A3  ; every char is in a word !
     mov (A3),D3
     add 0xffff,D3 ; test for zero
     brnc reset
     mov (A3),D0
     mov A3,D3 ; save A3 in D3
    ;-- function for char transmit --
    ; input char in D0
    ; return address N/A
    ; I/O register addr in A2
    ; first comes a startbit (low)
    ; then data, highest bit comes first
    ; then comes stopbit (high)
     mov 0xfff4,A3 ; bit counter
     shl D0
     shl D0
     shl D0
     shl D0
     shl D0
     shl D0
     shl D0 ; now shifted 7 times
     ; put start and stopbits
     add 0x007F,D0 ; stopbits high. startbit already ok.
    ; set data line high or low depending
    ;     on bit shifted out of registerD0
     mov D2,(A2+24) ; data low
     shl D0
     brnc bitlow
     mov D2,(A2+26) ; data high
    ; delay section
     movc 6,D2 ; bit timer
     add 4,D2
     brnc bitdly1
     mov D2,(A2+18) ; clk high 
    ; delay section
     movc 6,D2 ; bit timer
     add 4,D2
     brnc bitdly2
     mov D2,(A2+16) ; clk low
    ; count number of bits to transmit
     add 1,A3
     brnc bitloop
    ; small delay after character
     mov 0,D2 ; bit timer
     add 20,D2
     brnc dly1
    ; mov D0,PC ; ret
     jmp loop  ; do next character
     data section
    ; dw newline
     dw 'H'
     dw 'e'
     dw 'l'
     dw 'l'
     dw 'o'
     dw ' '
     dw 'W'
     dw 'o'
     dw 'r'
     dw 'l'
     dw 'd'
     dw '!'
     dw ' '
     dw 0

    I had to avoid the NOR instruction, that has an error in one of the 16 result bits. I expect that won't be difficult to trace. 

    There is still no RAM mounted. All the data required for the "Hello World" program could be kept in registers.

  • Blinking LED !

    roelh01/25/2020 at 19:37 0 comments

    A major milestone reached today.

    The Kobold K2, without debugger attached, runs a program from its two Flash ROM's, and blinks the LED on the main pcb ! 

    It runs on the RC-clock that is on the main PCB (U9A, HC132, 4K7 and 470pF). No RAM has been mounted yet.

     Here is the program:

    ; Kobold K2 
    ; Blinking LED
    ; 20200125
     movp 0,WP ; set wp and its page to zero
     movp 0x20,A2 ; address and page of output register
     add 4,D0
     brnc delay1
     mov D1,(A2+28) ; led on
     add 4,D0
     brnc delay2
     mov D1,(A2+30) ; led off
     jmp loop

    Some notes about the program:

    • The output is an addressable latch. Address bit A1 tells if the LED is on or off. So the value that is written to the address is unused, so there is no initializing of the D1 register needed. After a reset, this latch is reset and the LED is ON. So the LED is also a power indicator.
    • The loop counter in D0 is never initialized. After adding 4 enough times, there will be a carry and the delay loop ends. At that moment, D0 has a low value again and can be used for the next delay loop.
    • The WP (workspace pointer) is not used in the program, but it must be initialized otherwise the 8-bit immediate modes won't work.
    • MOVP 0x20,A2  will set A2 to 0x0020, and it's page will be set to the lowest 4 bits of the 0x20 value, so it's page is set to zero. (Similar for WP that is set to zero).

    It took about two weeks to reach this point. And of course, a job and other obligations limit the time that is available for the project.

    My desk during the debugging:

    Just a summary of the things I encountered. This is mostly for myself, if I loose my notes then I can find here what I did. The notes start before the previous log, the numbers are the item numbers in my notebook.


    A problem with the clock generator on the main pcb. The Kobold needs two clock signals, on the main pcb these are called CPU_CLK/ and CLK2/. CPU_CLK/ must get active first, and stay active for 75% of the time. CLK2/ comes later, is active for 50%, and both clocks should end around the same moment. This phase relation was not correct. After the correction, CPU_CLK/ had a spike, that was removed with a small RC network. The final correction to the schematic was:

    At pin 2 of U10A, a low-pass network 470E 33pF was inserted. U7B pin 11 was disconnected and connected to U7A pin 5.


    On the main board, one of the holes for the power connector was not big enough. 


    Created S, I, M commands in the RPi script for debugging, see previous log.


    During debugging, the clock coming from GPIO8 is inverted wrt the CPU clock. This is not a problem, but if you don't know this, you get strange results.


    The HC299 did not put its contents on the bus (see previous log). Disconnected pin 1 and connected it to its pin 2 (on both devices). The shift-left function of the HC299 is now no longer available, but is now done by storing the register at any position in memory. This has as side-effect that the ALU adds the data register to itself and stores the result in the same register. And, surprisingly, the opcode  doesn't even change. [edit: just found out the hard way, that the opcode does change. It is now the same as for a store instruction]


    Resistor R2 should get a more logical position on the pcb. Due to its strange position, I forgot to solder it.


    Started working on the Flash programmer. Starting point was the RPi Python script for the Risc Relay CPU. The RRC can only address words, while in the Kobold address bit A0 selects low or high byte, and is not connected to the Flash address bus. So the addresses had to shifted. Note that the RPi uses Kobold instructions to let the Kobold provide the correct address to the Flash. Both 8 bit Flash devices are programmed in parallel as if it were a single 16 bit wide device. The RPI provides the WE/ signal (on GPIO 25) to the flash to program it.


    The CE pulse...

    Read more »

  • Datapath working !

    roelh01/03/2020 at 16:17 4 comments

    Today I got the datapath working ! I mounted all parts, except those that are in the address generation section, because I wanted to start small. Also, no memory is mounted yet.

    As intended, the main board connects to a Raspberry Pi. At this moment, the connection has:

    • 16 databus lines
    • clock_enable ENA output to Kobold (GPIO24, low to disable Kobold clock)
    • clockpulse CLK output to Kobold (GPIO8, clockpulse is active high)
    • chip-enable CE output to Kobold (GPIO3. If high, Kobold and memory will not drive the databus)
    • EXEC signal from Kobold to RPi. Low for an exec cycle, high for a fetch cycle.

    The RPi can control the Kobold, that is needed for debugging and lateron for programming the flash. It will use the ENA output to stop the clock of the Kobold. It can then use the CLK to give a single clockpulse, and watch the EXEC signal to see if Kobold expects a Fetch or Exec cycle.

    In a fetch cycle, RPI must make CE high, then put an instruction on the databus, and give a clock pulse. The Kobold will place the instruction in the instruction register (U27 and U28) (with the "I" command in the Python script). The following exec cycle can be a read or write cycle. In a read cycle, the script must put data on the databus ("M" command in script) and give a clock pulse. The Kobold will think that it reads this from memory. 

    If the exec cycle is a write cycle, use the "W" command in the script. The RPi must make CE low, to enable the data output of the Kobold. It must also make its 16 datalines input. It can now give a clock pulse, and just before the end of the clock pulse it can read the data on its 16 datalines, and put that on screen.

    The following sequence is used in the Python script to add 1234 to 4321 :

    S       # stop Kobold clock
    I 6400  # fetch: instruction to load a word in register D0
    M 1234  # exec: provide the data that Kobold reads from memory
    I 4400  # fetch: instruction to add a word to register D0
    M 4321  # exec: provide the data
    I 9400  # fetch: write data from register D0 to memory
    W       # exec: Kobold writes result to memory, RPi catches 
                     it and displays 5555

    The Kobold has no addressing system yet. It is not needed for this test because the RPi handles all reads and writes, and also is agnostic of Kobold memory addresses at this moment.

    I also successfully tested the NOR operation and the data registers.

    It took a few days to reach this point. The HC299 has kept me busy. You could write data in it, but it always returned zeros. How difficult could it be ? Two inputs determine what happens at a clock pulse: Do Nothing, Load data, Shift Left or Shift right. Two enable inputs must both be low to put the contents on the bus.

    After a day of debugging, I read the datasheet word by word. And there is was: 

    Both output enable (OE1 and OE2) inputs are low and S0
    or S1 or both are low, the data in the register is presented
    at the eight outputs.

    So S0 and S1 must not both be high when you want output ! Always read your datasheets.

    After that, the system suffered from unstable behaviour. Sometimes it worked, and the next minute it didn't. This was finally traced down to my start-small strategy, where the address generation chips were not yet placed. I simply overlooked that a control signal that was needed for the data system, was routed through one of the missing chips. The signal was now undriven, that explains the strange behaviour.

  • Started soldering

    roelh12/29/2019 at 22:00 0 comments

    I started soldering the two boards. Here are the CPU and Mainboard, with sockets:

  • Online Javascript assembler/simulator

    roelh12/08/2019 at 19:13 0 comments

    The online assembler can now also simulate the Kobold K2 !

    The picture shows the panel with the source code, and the panel with the assembled code, and the first line highlighted (click for bigger version).

    You can run and singlestep. An integrated tiny disassembler shows the EA (effective address), the value at the effective address (or the immediate value VAL) just before executing the instruction.

    At the lower left, you see the highlighted instruction with opcode 0x6540 in the IR (instruction register), the effective address is 0x0042, and at that address is the value 0x0048. It also says that it will MOV that value to register D1.

    There is a new online instruction manual that shows all instructions and the addressing modes that are valid for them. When you activate the manual, it pops up at the side of the screen so you can still see and edit your program.

    And of course, there is a window that shows the text output of the simulation:

    Try it all LIVE, now near to you at Kobold K2 Assembler/Simulator !

    A free beer if you are the first to write a K2 program that generates primes !

  • PCB's ordered​

    roelh11/29/2019 at 21:17 0 comments

    Two PCB's are designed now:

    • The CPU
    • A mainboard for the CPU

    The video part is not designed yet. The mainboard has a 96 pin connector (same as CPU) where the video PCB can be placed once it is ready.

    Here comes the CPU impression from the PCB makers website:

    And the mainboard:

    The mainboard has:

    • DIN41612 connector for CPU
    • DIN41612 connector for Video card
    • Two 8-bit wide PLCC Flash ROMs
    • 256K x 16 SRAM
    • 32 MByte serial SPI flash
    • clock circuits
    • address decoding
    • some I/O
    • connector for Raspberry Pi for Flash programming and debugging
    • A software controllable LED
    • I/O connectors

    Schematics of CPU and mainboard are in the file section.

    Let the waiting begin.....

    In the mean time, I have time to order some parts.

  • Instruction Map

    roelh11/19/2019 at 16:17 0 comments

    After making many changes, it seems that the instruction set is now finally stable. The instruction set is now very orthogonal. An instruction map was made, that clearly shows the meaning of the upper 8 bits of the instruction (Click on it for a readable version. Excel file is in the file section).

    There are four addressing modes:

    • (An+d) Register indirect with 4-bit displacement, includes 16 bit immediates. The result can optionally be incremented by 1.
    • (zpage) One of 128 locations in the zero page (lowest bits in instruction)
    • An+d The value of an address register, incremented with 5 bit constant. Also used for short jumps.
    • Imm-8  an 8-bit constant (lowest bits in instruction)

    Almost all instructions write the result to one of the four 16-bit data registers, or one of the four address registers (this includes the PC).

    The most used instructions have a color in the map, from top to bottom:

    • Green, logical NOR and MOV-Complement.
    • Pink, 16-bit ADD
    • Blue, MOV instructions (including jumps)
    • Yellow, MOV to memory (store)
    • Orange, conditional MOV instructions (including conditional jumps)

    There are several empty positions in the map, so there is room for an extended version that has more instructions.

View all 23 project logs

Enjoy this project?



Pascal wrote 04/15/2020 at 10:46 point

Impressive project, and very interesting to read the logs. Keep it up!

  Are you sure? yes | no

threeme3 wrote 03/23/2020 at 20:14 point

Roel, congratulations with the progress. This is absolutely amazing now you got the k2 talking, and even have built a working c compiler for it! Wow! Really looking forward to reproduce your built and have a play with the c compiler. Are you planning to publish the compiler part, curious to see how it parses and generates the AST. All the best, Guido

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roelh wrote 03/23/2020 at 21:51 point

Thank you Guido ! I'll publish the compiler when it has become more stable. 

  Are you sure? yes | no

monsonite wrote 10/08/2019 at 10:06 point

Roelh - I like your minimum ALU and the use of the 74xx670 register file. I'm working on something similar, but using a 4-bit wide, bitslice approach in order to keep the logic layout and pcb design simpler. Are you proposing a 12.5MHz clock to keep things synchronous with a VGA output?   I'm looking forward to hearing of your progress.

  Are you sure? yes | no

roelh wrote 10/08/2019 at 10:34 point

Monsonite, I saw your postings on Anycpu/forum. For a CPU there are endless design possibilities...  I'm curious what you wil come up with. Yes, clock is synchronous with VGA. And thanks for for introducing Kobold on Anycpu:

  Are you sure? yes | no

monsonite wrote 10/08/2019 at 12:37 point

Roelh - I was intrigued by Kobold-1, now there are so many new ideas in Kobold-2. I have spent the morning reading the project logs so that I now have a better idea of your design, and how it works.  BTW - now that you can get very cheap 4 layer pcbs from China, I would recommend the use of separate power and ground planes.  This will improve your signal integrity, with faster edges, and reduce signal distortion and noise. As well as providing a very low impedance ground plane, you get much better power distribution and can eliminate the overhead of the wide power distribution traces on the signal layers.  The slightly increased cost will be justified by much improved performance.

  Are you sure? yes | no

threeme3 wrote 09/14/2019 at 10:38 point

Roel, very interesting development again. Just curious about the PC increment, looking to your drawing something very smart is happening there I think. Just guessing, is it that during the fetch cycle the current PC is incremented by the ALU (with one one of the values in the data registers) and  written back in A0?

  Are you sure? yes | no

roelh wrote 09/14/2019 at 11:14 point

The PC increment system is very unusual. I will soon write a log about it.

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Dave's Dev Lab wrote 09/14/2019 at 02:10 point

how are you planning to implement the VGA support?

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roelh wrote 09/14/2019 at 07:28 point

This will be similar to the first Kobold. But interrupts are difficult in the new design, so I plan to use a DMA system where the video system stops the CPU to obtain access to the shared RAM. So the CPU will only run during blanking time.

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Chase Rayfield wrote 09/18/2019 at 01:39 point

How about double clocking the ram and interleaving CPU and VIDEO accesses? I guess it depends on how fast the system clock is and how fast you ram is... even if only part of your ram used faster chips this might still make sense.

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roelh wrote 09/18/2019 at 07:30 point

Hi Chase, video will have to read two 8-bit pixels from memory every 80nS, and I don't think I will succeed making a 40nS cycle for video and a 40nS cycle for the CPU. But the CPU could run almost continuously if video got its own independent memory, or if only characters are read and an independent character ROM is used.

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Dan Maloney wrote 09/12/2019 at 14:54 point

Love these discrete chip CPU builds, especially TTL - I cut my teeth on those chips. Looking forward to seeing more progress!

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