🔔 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