Skip to content
/ zen-one Public

experimental retro 16 bit cpu written in verilog xilinx vivado intended for fpga cmod s7 from digilent

Notifications You must be signed in to change notification settings

calint/zen-one

Repository files navigation

zen-one

🔔 experiments continued in project riscv

experimental retro 16 bit cpu written in verilog xilinx vivado intended for fpga cmod s7 from digilent

third try at fpga with verilog and vivado

same toy 16 bit retro cpu as zen-x but re-written

  • ad-hoc pipeline where the next instruction is fetched while current is executed
  • ad-hoc pipeline resolution for register being loaded from RAM and used by current instruction
  • dual-port RAM instead of ROM and RAM
  • one cycle per instruction for ALU, load and store
  • two cycles for load immediate
  • two cycles for call and jump if branch taken
  • two cycles for instructions that also return from current call
  • all core components run on positive edge of the clock
                          
                          
                       z   n - r   c      vintage 16 bit cpu
                       e   e   e   a 
                       r   g   t   l      64K 16 bit ram
                       o   a   u   l      16  16 bit registers
                           t   r          64  calls stack
                           i   n          66  MHz
                           v              
                           e

                     | 0 | 1 | 2 | 3 |   4 - 7   | 8-11 | 12-15|        
                     |---|---|---|---|-----------|------|------|        
                     | z | n | r | c |   op      | rega | regb |        
                     |---|---|---|---|---|-------|------|------|--------
                     / . / . / . / 0 / 0 / 0 0 0 / .... / .... /  add  /
                    / . / . / . / 0 / 0 / 1 0 0 / .... / .... /  sub  / 
                   / . / . / . / 0 / 0 / 0 1 0 / .... / .... /  or   / 
                  / . / . / . / 0 / 0 / 1 1 0 / .... / .... /  xor  / 
                 / . / . / . / 0 / 0 / 0 0 1 / .... / .... /  and  / 
                / . / . / . / 0 / 0 / 1 0 1 / ...  / .... /  not  / 
               / . / . / . / 0 / 0 / 0 1 1 / .... / .... /  cp   / 
              / . / . / . / 0 / 0 / 1 1 1 / imm4 / .... /  shf  / 
             /---/---/---/---/---/-------/------/------/-------/ 
            / . / . / . / 0 / 1 / 0 0 0 / imm4 / .... /  addi / 
           / . / . / . / 0 / 1 / 1 0 0 / 0000 / .... /  ldi  / 
          / . / . / . / 0 / 1 / 0 1 0 / src  / dst  /  ld   / 
         / . / . / . / 0 / 1 / 1 1 0 / dst  / src  /  st   / 
        / . / . / . / 0 / 1 / 0 0 1 / .... / .... /       / 
       / . / . / . / 0 / 1 / 1 0 1 / .... / .... /       / 
      / . / . / . / 0 / 1 / 0 1 1 / .... / .... /       / 
     / . / . / . / 0 / 1 / 1 1 1 / .... / .... /       / 
    /---/---/---/---/---/-------/------/------/-------/ 
   / . / . / 0 / 1 /    immediate 12 << 4    /  call / 
  / . / . / 1 / 1 /   signed immediate 12   /  jmp  / 
 /---/---/---/---/---/-------/------/------/-------/ 
 \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 0100 \ .... \  wl   \
  \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 0101 \ .... \  wh   \
   \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 0110 \ .... \  rl   \
    \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 0111 \ .... \  rh   \
     \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 1100 \ .... \       \
      \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 1101 \ .... \       \
       \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 1110 \ .... \ led   \
        \ . \ . \ . \ 0 \ 1 \ 1 0 0 \ 1111 \ imm4 \ ledi  \
 

  op  :       : cyc |
 -----:-------:-----:--------------------------------------
 0000 : add   :  1  : reg[b] += reg[a]
 0010 : sub   :  1  : reg[b] -= reg[a] 
 0100 : or    :  1  : reg[b] |= reg[a] 
 0110 : xor   :  1  : reg[b] ^= reg[a] 
 1000 : and   :  1  : reg[b] &= reg[a] 
 1010 : not   :  1  : reg[b] = ~reg[a] 
 1100 : cp    :  1  : reg[b] = reg[a]
 1110 : shf   :  1  : reg[b] >>= (imm4>=0?++imm4:-imm4)
 -----:-------:-----:--------------------------------------
 0001 : addi  :  1  : reg[b] += (imm4>=0?++imm4:-imm4)
 0011 : ldi   :  2  : reg[b] = { next instruction }
 0101 : ld    :  1  : reg[b] = ram[a]
 0111 : st    :  1  : ram[a] = reg[b]
 1001 :       :     : 
 1011 :       :     :
 1101 :       :     :
 1111 :       :     : 
 -----:-------:-----:--------------------------------------

  rc  :       : cyc :
 -----+-------+-----+--------------------------------------
  01  : call  : 1-2  : pc = imm12 << 4
  11  : jmp   : 1-2  : pc += signed imm12
 -----+-------+-----+--------------------------------------

i/o
 |z|n|r|c| op |rega|regb| mnemo | description
 |-+-+-+-+----+----+----+-------+--------------------------------------
 |.|.|.|0|1100|0100|....|  wl   | uart blocking write lower regs[b]
 |.|.|.|0|1100|0101|....|  wh   | uart blocking write higher regs[b]
 |.|.|.|0|1100|0110|....|  rl   | uart blocking read lower regs[b]
 |.|.|.|0|1100|0111|....|  rh   | uart blocking read higher regs[b]
 |.|.|.|0|1100|1100|....|       | 
 |.|.|.|0|1100|1101|....|       | 
 |.|.|.|0|1100|1110|....|  led  | sets leds to lower 4 bits of regs[b]
 |.|.|.|0|1100|1111|imm4|  ledi | sets leds to imm4

zn flags in instructions are compared with current flags
instruction executes according to:
 * zn=11 : always
 * zn=00 : positive number
 * zn=10 : zero
 * zn=01 : negative number

instructions with rc=10 return from call

how-to with Vivado v2023.1:
 * to program device edit path to RAM file in "zen-one.srcs/sources_1/new/SoC.v"
 * connect fpga board Cmod S7 from digilent.com
 * run synthesis, run implementation, program device
 * find out which tty is on the usb connected to the card (e.g. /dev/ttyUSB1)
 * connect with terminal at 9600 baud, 8 bits, 1 stop bit, no parity 
 * button 0 is reset, click it to restart and display the prompt
 * "HELLO" is the prompt
 * after the prompt the program enters a read / write loop (echo)
 
programming zen-x
 * use 'zasm' to compile assembler code
 * see 'zen-one.srcs/sources_1/new/init.zasm' for examples

About

experimental retro 16 bit cpu written in verilog xilinx vivado intended for fpga cmod s7 from digilent

Topics

Resources

Stars

Watchers

Forks

Packages

No packages published