Kobold - retro TTL computer

A computer with 20 address bits, 16 bit instructions, video display, from just a few TTL and memory chips. Aiming for PDP11 instruction set.

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After having designed the one square inch ttl cpu, the moment comes that there is a new
minimal-parts computer that wants to be designed. The Kobold computer has chosen me to design him.

Its main characteristics are:
- fast 16 bit processor, 16 bit bus
- can access one Megabyte of memory
- 80 x 25 characters text, keyboard input
- full color mode and sound
- onboard mass storage 32MByte
- I/O connectors with Arduino layout

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

The drawing shows how the Kobold CPU (in SMD version) could be plugged into the mainboard of the Kobold computer system.

The end result will be available to everyone, either as completed product or as a kit.


It will consist of the following parts: 

  • processor in the spirit of the #1 Square Inch TTL CPU  
  • a simple ALU 
  • some address registers 
  • at least 256 KByte RAM 
  • flash-ROM for booting 
  • VGA text output 80 * 25 chars 
  • VGA color output (resolution TBD) 
  • PS/2 keyboard input 
  • user-accessible I/O pins (footprint of Arduino shield)
  • a sound system
  • serial flash (16 or 32 MByte) for storing all your programs. 

The text mode will have several (probably 4) colors, and the goal for video is freely mixing of text and full color mode.

At this moment, the idea is that the CPU and the video part will operate quite independent. The CPU might even be a separate pcb that is placed on top of a "motherboard", that has the memory, video and other I/O.

The CPU will probably be around 32 TTL parts and a microprogram Flash.


The instructions will be implemented in microcode. There will be two main instruction sets:

  • The native KOBOLD instruction set has highest execution speed and very good code density. The instructions are similar to PDP11 instructions. The instructions can access 1MByte of memory.
  • The K11 instruction set is binary compatible with the PDP-11 instructions. It is kept in mind, but will only be given full attention after most other goals are reached. It might become a separate project.

The microcode can be easily reprogrammed with a Raspberry Pi as programmer (just as with the 1 Square Inch TTL CPU ). The microcode Flash is large enough to accomodate a lot of microcode.


A Raspberry Pi can be connected, that can read or write files from or to the serial flash of the Kobold.

When enough supporting software is in place, the programs for Kobold can be developed on the device itself, perhaps first in BASIC and later in C. 


  • design the CPU
  • pcb design of CPU with thru-hole components
  • design the motherboard
  • pcb design of motherboard
  • some simulations
  • order boards, and assembly
  • hw fault finding
  • pcb design of a small Kobold CPU in SMD version
  • build or adapt a C compiler
  • make system software. Might adapt an OS.

This is a work in progress, the logs will show the design steps....

ms-excel - 35.50 kB - 05/26/2019 at 20:00



File to use with LOGISIM for simulating the micro-instructions

circ - 818.64 kB - 05/26/2019 at 19:59


Adobe Portable Document Format - 61.04 kB - 05/26/2019 at 19:59


  • Microcode simulation

    roelh7 hours ago 0 comments

    This weekend I simulated the microcode in Logisim (V2.7.1). A few things didn't work, so schematic changed a little. The files in the file section are updated.

    Some things were added in microcode:

    • Micro-instruction to clear Y register (overlaps with bit set/clr). For easy access to a fixed region (at address zero), for constants or for variables for the video interrupt.
    • Micro-instruction to clear upper byte of B register (overlap with other bit set/clr). Now we can move an immediate byte to a word register (Rn) with the upper half of the word set to zero, in a single 16 bit instruction.

    The microinstructions are simple, but a sequence of them will be quite capable. It will be possible to add the 'long' instruction type to our PDP-11 lookalike instructions, so 32 bit MOV, ADD (and more) can be handled ! 

  • New Microcode instructions !

    roelh6 days ago 0 comments

    The past weeks were spent designing the CPU schematic, and optimizing the available microcode instructions. The CPU schematic is now in the file section.

    As a reminder, here are the hardware registers:

    The registers A and B are always loaded at the same time (with the same data). The B register contents can be stored in memory (and can also be shifted left or right). The A register is connected to the input of the ALU.

    The UPC (micro-programcounter) register will contain the 'user' instruction. Two 3-bit fields can be used to select a source- and destination register. The remaining 10 bits select an address in the microcode storage. 

    There are 3 classes of microcode instructions. The first one is register load/store:

    The user registers R0-R7 are addressed as (WP+displacement). For the displacement, the source or destination register field in UPC can be chosen, these will be at addresses 0,4,6,8,A,C,E. The microinstruction has the option to add 2 to the displacement. This makes it possible to address 32 bit data (in memory or WP-based registers) in two 16-bit chunks, or address 16 word-sized registers instead of 8.

    Note that when the instruction is loaded  into UPC, it is also loaded in AB and Y. Why ? When the instruction contains immediate data, it is directly available to the ALU to do calculations. If it is an immediate byte load, the byte is already in the B register and can directly be moved to the destination.

    The destination register can not be the same as the pointer because pointer register is a latch and not a edge-clocked flipflop.

    The RL register is used to temporary store the result of the ALU for low bytes. When the ALU calculates the high byte, the high byte will be combined with the low byte that was stored in RL, and the 16 bit result will be stored at the destination.

    The PC can also be input to the ALU. This is useful for adding a constant to the PC (branch) or for obtaining the return address when doing a CALL instruction.

    All microinstructions can be executed conditionally depending on carry, zero or other conditions (Actually, at each microstep a certain microinstruction is selected depending on conditions).

    There are 16 output signals that can be individually set or cleared by microcode. These are used for:

    • selecting the condition for conditional execution
    • put the B register in a mode for shift left or shift right
    • write only to upper or lower half of a memory word (for byte writes)
    • video signals, like pixel count reset, sync outputs
    • several other I/O

    [edit: instruction codes updated 20190526]

  • First PCB routing

    roelh04/30/2019 at 19:56 0 comments

    Just a quick test to see how it would fit on a PCB and if it would be difficult to route. This is the CPU only. It is connected to the mainboard by the two 2 x 18 headers. Bypass caps are not yet placed.

    Routing was easy, the autorouter did it within a minute ! That is, after experimenting with placement a few hours. The size is about 100 x 150 mm.

  • Schematic of the new ALU

    roelh04/27/2019 at 18:53 0 comments

    The biggest parts of the schematic of Kobold are almost completed. Here you see:

    • Register B, that holds 16 bit data, and can shift the data left or right, and put it back on the databus
    • Register A, that holds 16 bit data and can put a single 8-bit chunk on the address bus. The other 8 bits of the address bus will be provided by an address register (not shown here). During ALU operation, the address bus will not be used to provide an address to memory.
    • The logic unit performs a NAND on the two 8-bit halves of the address bus
    • The Adder adds the two 8-bit halves of the address bus
    • The function selector selects the NAND or the ADD as result. Microcode bit IR4 is used to select the function.
    • Finally, the 8-bit result goes to the lower or upper 8 bits of the result register
    • The result register can put the 16-bit result on the databus

  • Plan for video generation

    roelh04/26/2019 at 19:05 0 comments

    Trying to have the hardware simple, and keeping in mind that 'large' SRAMs are easy to obtain, the choice was made to have just a single basic video mode.

    That is, 640 x 480 pixels, with 256 colors per pixel. The pixel time for VGA is 40nSec.

    The hardware needs assistance from software in order to operate correctly.

    The pixel counter will increment every 80nS, so the video RAM will deliver two new pixel values every 80nS. The 8 bit color values go to the 'first pixel' and 'second pixel' registers. During the next 80nS, each of the outputs of these registers will be enabled for 40 nS.

    When the pixel counter has reached a certain value, an interrupt will be given to the CPU. In the interrupt, the CPU will:

    • obtain exact synchronization with the pixel counter
    • when the pixel counter is exact 199, reset the pixel counter to zero, to obtain 32 uSec line duration
    • start the horizontal sync signal
    • obtain access to the video RAM, disabling the pixel counter and the updating of the two pixel color registers, and enabling the pixel(X) register. It can now, after setting the correct pixel address in the line and pixel registers, write new data to the video RAM. The line and pixel registers may be latches, so the data can be written in the same cycle together with the address. Several locations may be written, depending on the available time during blanking.
    • set the line (Y) register to the starting point of data for the next line
    • when the end of a frame has been reached, do functions for frame synchronisation
    • stop the access to video RAM, and stop the horizontal sync signal
    • end the interrupt

    The sequence of actions might be a little bit different than listed here. 

    The actions might be done by microcode, and the 16 bit processor can deliver 16 bit at a time, so this will be fast enough for most operations. If there are applications (games) that need higher speed, resolution could be dropped to 320 x 240 (TBD to be determined).

    Since the starting point of a line is under software control, it will be easy to do fast vertical scrolling. A clear screen will also go fast, because only a single cleared line has to be present, and all other lines can point to the same cleared line.

  • System diagram

    roelh04/21/2019 at 09:13 0 comments

    This is the new system diagram for the 16 bit bus system.

    The program counter is in the set of four registers. However, incrementing the program counter would either cost several extra cycles or extra hardware. In this low-part-count solution, the lowest 4 bits of the PC are in a hardware counter, and the other bits are provided by the register set. When the 4-bit counter overflows, the microcode will add 16 to the part that is in the register set.

    Register B is a temporary register for MOV instructions. It is loaded with 16 bit data from a source memory location, and then its contents can be stored to a destination memory location. These locations include the R0-R7 because they are also in memory. Register B can also shift data. It is constructed of two 74HC299 devices.

    The ALU can do only two instructions, ADD and a logical instruction, NOR or NAND, [TBD, to be determined]. Other instructions are done by microcode. The ALU takes one operand from one of the registers PC, WP, X or Y, and the other operand from A. The outputs of  these registers can be tri-stated in 8-bit sections, so adding A to X will first enable the low section of X and A, add them and put the result in the low section of R. Then the same will be done for the high byte. 

    Note that the ALU can also swap the two bytes in a word. This will be used when byte instructions need to store a byte in the upper 8 bits of a memory word. The microcode will look at address bit 0 to determine if a byte must be moved to or from the low or high part of a memory word. For storing a byte to a memory location, the microcode can disable writing to one of the two bytes that  are in a memory word, so when a byte is written, the other byte in that memory word is not affected.

  • Design upgrade

    roelh04/18/2019 at 11:27 1 comment

    The current design of the microcode structure is almost complete, with just a few loose ends that have to be solved.

    However, the total design effort for the project will be quite huge, and it would be a pity if the result is inferior on certain aspects. It seems that with a little more effort, a better result can be obtained (at the cost of a few more components).

    This will focus on two main aspects:

    • Most instructions are just moving data around. If we change the bus from 8 to 16 bit, the system will run almost twice as fast. The ALU might 
      stay 8 bit to keep the part count reasonable (This will perhaps only cost a single extra cycle for instructions that do calculations).
    • Several aspects of the Kobold are inspired on the PDP11. With some more effort, the design could be such that the microcode can implement a binary 
      compatible PDP-11 instruction set. (This will only be practical with a 16 bit bus).

    So, work to do !

  • Opcode handling

    roelh04/15/2019 at 17:17 3 comments

    The Kobold is advertised to handle 16 bit instructions, but everything is 8 bit. Even the microcode is only 8 bit wide. How does it work ?

    [ What you need to know for the examples: The program counter is in register R7 (as on the PDP11), and is copied into address register PC when needed. The lowest bit of the PC is always 0.]

    EXAMPLE: 16-bit ADD

    As example, take an instruction that adds (X+6) to register R4:

     add (X+6),R4 

    The instruction is split into two parts, that operate almost independent of each other:

    The first byte of the instruction is fetched from (PC) into the micro-program counter. From here, the micro-instructions determine the operation:

    • First part:
    • load the LSB of the 16bit accumulator with (X+6), (the LSB part)
    • load the MSB of the 16bit accumulator with (X+6),  (the MSB part)
    • fetch an instruction byte from (PC+1) into the micro-program counter (changing flow of the microcode). 

    This instruction byte tells to add R4 (16 bits) to the accumulator and store the result back to R4:

    • Second part:
    • add the LSB of (WP+4) to the LSB of the accumulator
    • add the MSB of (WP+4) to the MSB of the accumulator
    • store the LSB of the accumulator back into (WP+4) (the LSB part of R4)
    • store the MSB of the accumulator back into (WP+4) (the MSB part of R4)

    The next section of the microcode will increment the PC and start the next instruction:

    • connect the LSB of the PC (from the address registers) to the B-input of the adder of the ALU. The value 2 will be provided to the A-input of the ALU, so the byte value P+2 will be put in the accumulator.
    • fetch the MSB of the pc from R7 [in the workspace] and put it into the accumulator, to make 16 bits complete.
    • move the 16-bit accumulator contents to the pc in the address register set.
    • fetch the next instruction from (PC) into the micro-program counter.
           ( from here, the micro-instructions for the next instruction are executed )

    EXAMPLE: 8 bit immediate load

    There are also instructions that have a single opcode byte, followed by an 8-bit immediate operand or z-page location. Branch instructions are  an example of this.

    As example, load register R3 with value 0x80:

    mov #0x80,R3
    • load the LSB of the accumulator from (PC+1)
    • load the MSB of the accumulator with zero
    • store the LSB of the accumulator in (WP+3)  (the LSB part of R3)
    • store the MSB of the accumulator in (WP+3)  (the MSB part of R3)

    Finally, the pc is incremented and the next instruction is fetched, as in the previous example.

  • Address registers schematic

    roelh04/14/2019 at 18:17 0 comments

    Here you see the schematic of the address registers, with the five HC670 chips. 

    Loading the address register

    The inputs (REG0-REG15) come from the accumulator. The address register to write to is selected with the WA and WB inputs (that connect to bit IR4 and IR5 of the microcode byte). When ADDR_WE is active (low) the data is written into the selected address register. 

    The upper four bits (16-19) of the address register are written when ADDR_PAGE_WE is active.

    Output of the address registers

    The five 670's always put an address on the address bus (A0-A19), because the GR signal is always active. When the USE_XY signal is low, the PC or WP is connected to the bus (selected with bit IR4 from the microcode). When USE_XY is high, the X or Y register is connected to the bus, also selected with IR4. 

    Adding the displacement

    At the upper right you see five OR-gates that "add" a displacement to the address. The lower four bits of the displacement come from the microcode byte. The fifth bit (DISPL4) comes from the control section. Since the displacement is not really added, it only works if the address in the address register is properly aligned. Alhough a four bit adder chip could have been used for the A1-A4, this only moves the problem when that adder has a carry. But having more than one adder chip here is against the minimum parts philosophy of the project.

    The upper signals AP0-AP4, together with A5-A8 can be connected to the adder in the ALU. This connection is used for incrementing the PC. Only the lower byte of the PC is incremented. To cross a 256-byte boundary, an explicit instruction will be needed.

  • The ALU of the Kobold

    roelh04/12/2019 at 20:01 0 comments

    This is the current state of the ALU design. There are still several loose ends. Carry signals for addition and shifting are not yet present. The ALU is 8 bits wide, but for clarity I only show 4 bits. (Clicking it will show a better readable version).

    The ALU functions are LOAD, ADD, BIS, BIC, SHR and SHL

    The ALU is intended to work on 16 bit words, in two sequential cycles that each handle a byte.

    ADD function

    The ADD function is the easiest to explain. The upper MPX (multiplexer) connects the output of the AL accumulator byte to the upper inputs of the adder chip, 74HC283. The lower MPX is disabled (by ALU_F0 signal), so all its outputs are high. That means that the input from the databus (D0-D7) is flowing through the AND-gates to the lower inputs of the adder chip. So, the adder will add the databus byte to the AL byte and deliver the result in AH.

    Ooops... what is that lower byte doing in the high part of the accumulator ?  And the accumulator has been clocked, so the high byte of the accumulator is now in the low byte ?

    After the next cycle, it will be all right. The high byte coming from the databus will be added to the high byte of the accumulator, that is in AL now. The result will go to the AH register, and the previous result of the low byte will at the same time go to the HL register.

    LOAD function

    How do we put something in the accumulator ? We set the ALU-F1 signal (connected to the upper MPX) to 1. This disables the upper MPX, its output will be zero. We now do the ADD operation. The databus contents will be added to zero, and the result will be put in the accumulator.

    BIC function

    For the BIC (bit clear) function (that is a logical AND where one of the operands is inverted), the upper MPX is disabled but the lower MPX is now enabled. The lower MPX inverts the data (it is a 74HC158). The following AND gates will give result:  DATA and (not ACCU). The adder will add zero to this, so that does not change the result. So the result is that any bit that was set in the accumulator, will cause the same bit from the databus input to be cleared.

    BIS function

    The BIS (bit set) function (PDP11 parlance for logical OR) is similar to BIC, but now also the upper MPX is enabled again. so both adder inputs can receive data. When an accumulator bit is 0, the AND gates will transfer the databus inputs, the adder will add the 0 bit to it, so result for this bit is 1 (if there was no carry from previous bit). And when an accumulator bit is 1, it is inverted by the lower MPX so it delivers 0 to its AND gate, so the lower input of the adder will be 0.  The adder output will be 1 because the upper input is 1 and the lower input is 0. The key idea is, that at the adder, the two inputs for a certain bit are never both 1. In this case, the adder will never generate an (internal or external) carry, and will behave as four OR gates.

    SHR function

    Finally, the SHR (shift right) function can be done by the upper MPX, because it has a shifted version of the accumulator bits connected to one of its input groups. The databus input should be zero (or not, in that case it will be added to the result).

    SHL function

    SHL (shift left) is the same as adding.

    Other functions

    A few functions are missing, but they can be composed from the functions that are available. This can be done in microcode, so the instruction set can still contain these missing functions. It will only have a small impact on performance. The functions that I'm talking about are SUB, XOR and NOT.

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Enjoy this project?



Marcel van Kervinck wrote 04/28/2019 at 12:21 point

I like where the Kobold design is going. I wasn't aware of the 74LS670 register file, and I'm positively surprised by its availability. Although it has few pins, I think I would have considered it too complex for the Gigatron objective had I been aware of it. It has a lot more transistors than the '181 for example. But not many pins, so that's cute.

  Are you sure? yes | no

bobricius wrote 04/14/2019 at 17:07 point

I am exited with this project.

  Are you sure? yes | no

roelh wrote 04/10/2019 at 12:48 point

Yes, you made it famous, thank you !

  Are you sure? yes | no

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