The shmerlard 16

hi, my name is elad and Im an electrical engineering student in Ben gurion university. I designed a small 16bit cpu, with a mix of jdh8, src, msp430 and the Hack computer. this is still a work in progress, but so far it works.

I used logisim for the logic design, draw.io for the diagrams. in order to see the working cpu in logisim: you need to build version 3.9.0 from source here, then just open the .circ file.

Specs

• a common 16-bit data and address bus
• a total of 128kB of usable memory
• 6 GP register (up to 14 available)
• modular design for easy troubleshooting
• port for outside connections

Contents

1. Registers
2. Instruction Register
3. Program Counter
4. ALU
5. Control Unit
6. Memory Registers
7. Condition Logic
8. Shift Control
9. Gallery

Instruction set

0x00:  NOP
0x01:  MOV  Ra/#, Rb       | Ra/# -> Rb
0x02:  ST   Ra/#, Rb/#     | Ra/# -> M[Rb]/
0x03:  PUSH Ra/#           | Ra/# -> @SP; SP--
0x04:  POP  Ra             | SP++; @SP -> Ra
0x05:  ADD  Ra/#, Rb       | Ra/# + Rb -> Rb
0x06:  ADDC Ra/#, Rb       | Ra/# + Rb + c -> Rb
0x07:  SUB  Ra/#, Rb       | Rb - Ra/# -> Rb
0x08:  CMP  Ra/#, Rb       | Rb - Ra/#
0x09:  SHR  Ra/#, Rb       | SHR Rb (Ra/# times) -> Rb
0x0A:  SHL
0x0B:  RRC  Ra/#, Rb       | RRC Rb (Ra/# times) -> Rb
0x0C:  RLC
0x0D:  AND  Ra/#, Rb       | Ra/# AND Rb -> Rb
0x0E:  OR   Ra/#, Rb       | Ra/# OR  Rb -> Rb
0x0F:  XOR  Ra/#, Rb       | Ra/# XOR Rb -> Rb
0x10:  NOT
0x11:  CALL DST            | PC -> @SP; DST -> PC; SP-- -> SP;
0x12:  RET
0x13:  JMP  DST            | DST -> PC

for more simulated instructions see: Simulated Instructions

Registers

0: SP/REG0  : STACK pointer    
1: SR/REG1  : Status register
2: REG2     : GP Input
3: REG3     : GP Output
4: REG3     : GP
5: REG4     : GP
6: REG5     : GP
7: REG6     : GP

Memory

the memory ranges from 0x0000 to 0xFFFF. the memory is mapped in the following way:

0x0000 - 0x7FFF: GP ROM              (32KiB    ROM)
0x8000 - 0xBFFF: GP RAM              (16KiB    RAM) 
0xC000 - 0xCFFF: VRAM                (4KiB    RAM) #CURRENTLY NOT IN USE#
0xD000 - 0xFFFF: NOT IN USE          (12KiB UNUSED)

Instruction format

The instruction register is a 16-bit register

the format is XXXX-XUMM-SSSS-DDDD
X = the opcode of the instruction
U = Currently not in use
M = indicates the addressing mode
S = indicates the source register
D = indicates the destination register

(see addressing modes) for more info

Full diagram

for the full gallery go to Gallery