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Kobold K2 - RISC TTL Computer

A 16 bit RISC computer with video display, from just a few TTL and memory chips.

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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 74181 ALU

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

MOTIVATION

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 almost twice as fast, and its operation will be easier to explain.
The video system will stay mostly the same.


STRATEGY

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.

PROGRAMMING ENVIRONMENT

To make programming easy, an Online Javascript Assembler/Simulator was made. The assembly code for the K2 processor can be made and assembled in your browser. It can also simulate the cpu. If you open the simulator just press Assemble and Run to see the Hello World 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....

PCB IMPRESSION

The pcb of the CPU is now (5 oct 2019) routed. It also gives an impression of the various CPU parts (ALU, Registers, Control), see also this log about the PCB. The final PCB has some changes.

LOGS

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 !

KMAIN1948 - Kobold Main.pdf

Kobold K2 Mainboard Schematic

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

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KCPU1948 - Kobold K2 CPU.pdf

Kobold K2 CPU Schematic

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

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

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

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hello 20191115.dmp

Hello World binary, to put in the Logisim RAM

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

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  • Blinking LED !

    roelh13 hours ago 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
    
    start:
     nop
     movp 0,WP ; set wp and its page to zero
     movp 0x20,A2 ; address and page of output register
    
    loop:
    
    delay1:
     add 4,D0
     brnc delay1
    
     mov D1,(A2+28) ; led on
    
    delay2:
     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.

    1)

    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 330pF was inserted. U7B pin 11 was disconnected and connected to U7A pin 5.

    3) 

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

    6) 

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

    7) 

    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.

    8)

    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.

    10)

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

    12)

    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.

    13)

    The CE pulse from the RPI (Called DATA_EN/ on the main board) doesn't reach the Flash. It seems this can only reach the...

    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.

  • Hello simulator !

    roelh11/15/2019 at 15:53 0 comments

    Today a minor milestone was reached.

    The javascript assembler is working (for most instructions). I didn't work yet on the Javascript simulator .

    But I also have a Logisim simulator. The assembler output for 'Hello World' was loaded in the Logisim simulator, and it worked ! It's the first real running program written in Kobold Assembly !

    The program is this:

                0 ; Kobold K2 assembler test
                1 
                2 screen: equ 0xf000
                3 newline: equ 0x0d
                4 
    00000 7C00  5  mov 0,d0
    00002 7A20  6  mov text,a2
    00004 631E  7  mov screen,a3
    00006 7008+ 8 loop:
    00008 6540  9  mov (a2),d1
    0000A 7242  10  add 2,a2
    0000C 9560  11  mov d1,(a3) 
    0000E 4514  12  add 0xffff,d1 ;test for zero
    00010 A092 b13  brc loop ; branch if non zero
    00012 700C+ 14 hlt: jmp hlt
    00014 6084 
                15 
    00016 AAAA  16  data section
    00018 0014-
    0001A 0008-
    0001C FFFF-
    0001E F000-
                17 
                18 text:
    00020 0048  19  dw 'H'
    00022 0065  20  dw 'e'
    00024 006C  21  dw 'l'
    00026 006C  22  dw 'l'
    00028 006F  23  dw 'o'
    0002A 0020  24  dw ' '
    0002C 0057  25  dw 'W'
    0002E 006F  26  dw 'o'
    00030 0072  27  dw 'r'
    00032 006C  28  dw 'l'
    00034 0064  29  dw 'd'
    00036 000D  30  dw newline
    00038 0000  31  dw 0

    Some remarks:

    • In the instructions, source comes before destination: MOV SRC,DST
    • The first instruction (mov 0,d0) is non-functional, because execution after reset skips the first instruction
    • There is no HLT instruction. A the end of the string, the program just keeps jumping to the same HLT label.
    • Note the use of the ADD 0xffff instruction to test for zero. That's because there only is a carry flag, no zero flag.
    • Immediates that do not fit in 8 bits are 16 bits and placed at the end of a 16 word chunk, and flagged with a '-' for the human reader. so you find the 0xffff for the 'test for zero' on address 0x0001C. The values 0014 and 0008 are for branches to loop and hlt, to be discussed later.
    • The logisim textscreen responds to writing to every address of 0x8000 and higher. The textscreen has the address 0xf000 in this example.
    • The Logisim simulator can not read bytes from memory, so the text has been placed in words. The actual CPU wil be able to read bytes.
    • Line 13 is marked 'b' and lines 8 and 14 are marked '+', both will be explained later.
    • Actually, the page registers of A2 and A3 should have been set to 0 with an instruction, but in the simulator they are already zero by default. However, the reset hardware does set the page of PC to zero. 

    The Logisim file and the file to be loaded in the Logisim RAM are in the file section. 

    The assembler can be tried here: Kobold K2 Assembler. Just press 'Assemble' to run the assembler. Feel free to try some code changes. The assembler can load/store files from/to your own PC.

  • New instruction set

    roelh11/03/2019 at 13:57 0 comments

    Here is the new instruction set. It is quite conventional (thats my personal view), but has a few quirks due to the fact that a lot of functionality was pushed into the instruction set, while keeping the decoding circuits very simple.

    I will present the new set in three encoding tables, starting with a few simple ones.

    (There is also a colored instruction map available)

    WRITE TO MEMORY

    There are two ways to address memory:

    • Pointer with displacement. WP, A2 and A3 can be used as pointer (PC should be used for reading only).
    • Zero page. Zero page size is 128 words (256 bytes).

    All four data registers can be written to memory, either as word or as byte. To write an address register to memory, first move it to a data register and then write it.

    INCREMENT ADDRESS REGISTER

    Address registers can be incremented with a value from 1 to 31. This can also be conditionally, mostly used in combination with A0 (PC). So the branches can skip 15 instructions at most. For greater distances, 16-bit immediate conditional jumps must be used. Note that the destination of the addition can also be a data register.

    ALL INSTRUCTIONS

    This may look a bit complex. But you just pick an instruction, a source and a destination.

    Note that there are several ways to specify the source operand:

    • Address register indirect with displacement, (includes 16-bit immediate)
    • Zero page location
    • Address register plus small (5 bit) constant
    • short immediate (7 or 8 bit)

    Oddities:

    • The MOV to register instructions can be conditionally executed.
    • A jump or branch is just a move or conditional move to the PC
    • There are MOV-with-increment instructions. One of its uses is incrementing a value in memory with just two instructions
      • MOVI (value),D2 ; get value in D2 and increment it
      • MOV D2,(value) ; store value back
    • ADD-with-increment is just adding with the Carry-input of the ALU set. Useful for subtraction.
    • For MOV (and ADD) with 8-bit immediate, the lowest immediate bit is formed by choosing between the MOV and MOVI instruction. The assembler will handle this. Since this lowest immediate bit is connected to the carry-input of the adder, it is not available for the logical functions NOR and MOVC.
    • SHL and SHR instructions act on the result of the previous instruction.
    • The SHL instruction writes to memory as a side-effect. The assembler will use the highest zero-page word (at 0x00FE) as location to write to.

    Remarks for address register destination:

    • ADD instructions have 3 operands, the source-2 operand is always a data register, and the number of the source-2 data register must be the same as the number of the destination address register. So ADD A1, D2,A2 is possible but ADD A1, D2,A3 is not possible.

    [ edit 20191115: I pushed some more functionality in the ISA, schematic in file section has been updated ]

    [ edit 20191119: new schematic uploaded to file section ]

  • More conventional instruction sequencing

    roelh11/01/2019 at 22:02 0 comments

    This week I was working on the assembler again. As said in the previous log, the assembler is quite complex because it also has to place the instructions in the correct sequence. 

    Although the instruction sequencing and the conditional branching can be explained, the inner working of the assembler will be obscure due to its complexity.

    And if we ever come to the point where programs can be built on the machine itself, we also need an assembler that runs on the machine itself, so that must probably also be written in assembler (while the current assembler is written in Javascript). I shivered at the thought of having to code this again.

    So I decided. Design change.

    Thats the nice thing about a hobby project. You can keep changing. Your project can even keep changing without ever coming to an end....

    The instructions wil be in sequence. There will be a hardware 4-bit program counter, that will address the instructions together with the other bits in the HC670 register, much like the Kobold-one.

    At the end of a 16-instruction block, a jump instruction must be placed to go to the next block.  This will be done automatically by the assembler. Conditional branches will now also be done in a conventional way. 

    The three NNN bits that hold the next slot number can now be used for something else.  This will make the decoding of the special instruction variants easier, and give room to provide more options for some instructions.

    [ edit: In fact, the instruction variants vanished, making the ISA much more orthogonal. There is now also room for 8-bit immediates within the 16 bit instruction, and zero page size expanded to 128 words, see next log. ]

  • Changing the memory access model​

    roelh10/16/2019 at 17:52 0 comments

    In the past weeks, I did first draw the design in the Logisim simulator. The first few instructions were succesfully simulated. Then I started working on a Javascript assembler-simulator combination.

    While I've been making assemblers in the past, this one proved quite difficult. You'll remember from one of the previous logs, that the instructions can be arranged within 8-instruction blocks in almost arbitrary ways. But the actual sequence becomes important for flow control and optimization of (conditional) jumps. Above that, each 8-instruction block must end with a jump to the following block, and sometimes a slot in a block stays unused because there is a multi-word instruction or instruction sequence that can not be distributed over two blocks.

    The goal was, that the assembly programmer doesn't have to concern himself with the above subtleties, and that the assembler program does all this. Now in a 'normal' assembler, the instructions have very less interaction with each other. That's totally different now. 

    But now that the struggle to do automatic instruction sequencing by the assembler has almost been completed, an inconvenience in the design came to the surface of my mind. That is the memory access system.

    The proposed system has two models, the linear and the object model. This forms a kind of two-dimensional memory system with the A0-A15 address on one axis, and the page-or-displacement value on the other axis. If a language like C would do memory allocation, using the Kobold system, that would mean that memory would be allocated in two dimensions. This would imply a lot of complications.

    So I decided to change to a more common model, while keeping most of the advantages of the 'old' system. The schematic of the old model is still available as version 20191011.

    NEW MEMORY MODEL

    In the new memory model, the object system has become a part of the linear model. It is almost the same as in the Kobold-one.

    The address of an operand is formed by:

    • A0: from address register. Is high for byte-access to the MSB of a word.
    • A1 - A4: from address register, OR'ed with the four displacement bits
    • A5-A15:  from the address register
    • A16-A19: 4 bits from the page register that belongs to the address register

    An instruction can have a 4-bit displacement that is OR'ed to bit A1 - A4 of the above address. The result determines the position of a memory operand.

    The address of the next instruction is constructed as follows:

    • A0 is always zero, because an instruction is a word.
    • A1, A2, A3 come from the NNN bits in the current instruction
    • A4 - A15 come from address register A0 (PC, program counter)
    • A16 - A19 come from the page register of the program counter

    You see that the lowest four bits of the program counter are not used to address the next instruction. That opens the possibility to store a copy of the program page in those four bits. This gives us the same more-than 64K jump capability as in the old model, for instance for return address storage:

    • The PC is moved to a dataregister, and the subroutine stores it in the stack frame
    • At return, the stored address is written to the program counter, and to the page register of the program counter, at the same time! As before, the return address is always in the same instruction slot.
    • So now the page register has correct contents, because that was previously stored in A0 - A3. And A4 - A15 will be used to fetch the instruction, together with the page (A16 - A19).

    And, also, a jump or call is still able to reach all memory positions without a near-or-far mechanism.

    The schematic is now updated. The log Addressing modes was also updated.

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Discussions

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: http://anycpu.org/forum/viewtopic.php?f=23&t=623

  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?

  Are you sure? yes | no

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.

  Are you sure? yes | no

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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