Z16 Instruction Set Architecture (ISA) Specification

Z16 is a compact, 16-bit RISC-style instruction set architecture inspired by RISC-V RV32I, designed for small, efficient embedded systems. It supports 64 KB of byte-addressable memory and uses a fixed 16-bit instruction width. The ISA is optimized for simplicity and compact encoding, using a 2-register format for most operations.

Key Features

• Instruction Width: 16 bits (fixed)
• Registers: 8 general-purpose registers (x0–x7), each 16 bits wide
• Memory: 64 KB (16-bit addressing), byte-addressable
• Word Size: 16 bits
• Register Format: For arithmetic/logical instructions, the first register (rs1) is both source and destination
• All branches and jumps are PC-relative, except for JALR
• Immediate instructions support up to 7-bit signed immediates
• li (load immediate) is a true instruction

Register ABI

Reg	ABI	Function
x0	t0	Temporary register
x1	ra	Return address
x2	sp	Stack pointer
x3	s0	Callee-saved register
x4	s1	Callee-saved register
x5	t1	Temporary register
x6	a0	Function arg / return val
x7	a1	Function arg / return val

Comparing RV32I to Z16

	RV32I	Z16
Architecture	32-bit RISC ISA	16-bit RISC ISA
Memory	Byte addressable 4 Gbytes	Byte addressable 64 Kbytes
Registers	32 x 32-bit registers	8 x 16-bit registers
x0	zero	t0
li, bz, bnz, jr and j	pseudo instructions	true instructions
immediate	signed 12 bits	signed 7 bits
L/S Offset	signed 12 bits	signed 4 bits

Instructions Encoding

Instruction Types

1. R-Type (Register-to-Register)

• Opcode: 000
• Format: funct4 | rs2 | rs1 | funct3 | opcode
• Operation: rs1 = rs1 op rs2
• Examples: ADD, SUB, SLL, SRL, AND, OR, XOR, SLT, SLTU, SRA, JR, JALR, MV

2. I-Type (Immediate)

• Opcode: 001
• Format: imm[6:0] | rs1 | funct3 | opcode
• Operation: rs1 = rs1 op imm
• Examples: ADDI, SLTI, ORI, ANDI, SLLI, SRLI, SRAI, LI

3. B-Type (Branch)

• Opcode: 010
• Format: imm[4:1] | rs2 | rs1 | funct3 | opcode
• Examples: BEQ, BNE, BZ, BNZ, BLT, BGE, BLTU, BGEU
• Note: BZ and BNZ only use rs1, compared to zero

4. S-Type (Store)

• Opcode: 011
• Format: imm[3:0] | rs2 | rs1 | funct3 | opcode
• Examples: SB, SW

5. L-Type (Load)

• Opcode: 100
• Format: imm[3:0] | rs2 | rd | funct3 | opcode
• Examples: LB, LW, LBU

6. J-Type (Jump)

• Opcode: 101
• Format: f | imm[9:4] | rd | imm[3:1] | opcode
• Examples: J, JAL

7. U-Type (Upper Immediate)

• Opcode: 110
• Format: f | imm[15:10] | rd | imm[9:7] | opcode
• Examples: LUI, AUIPC

8. SYS-Type (System)

• Opcode: 111
• Format: service[11:0] | funct3 | opcode
• Examples: ECALL

Special Semantics

• All arithmetic/logical operations use 2-register format:
  rs1 = rs1 op rs2
• LI is a real instruction (not pseudo)
• JR and JALR jump to the address in a register (rs2)
• Jumps and branches are PC-relative
• Loads/stores use base+offset addressing

Calling Convention Summary

• Arguments: a0 (x6), a1 (x7)
• Return value: a0 (x6)
• Return address: ra (x1)
• Stack pointer: sp (x2)
• Callee-saved: s0 (x3), s1 (x4)
• Caller-saved: t0 (x0), t1 (x5), a0, a1

The Assembler

The repo contains a simple 2-pass assembler for Z16 ISA. The assembler is very similar to RISC-V ones. At the moment, the assembler supports only Z16 true instructions. The recommended assembly program skeleton is given below:

# Add some comments to describe the program

# the TEXT Section
    .text
    .org    0
main:
    # Your code goes here
    # you can use the ABI register names
    # Also, you may end the line with a comment

    li      a0, 25      # A sample instruction

    # A sample label
    # A label cannot be followed by anything
L1:

   # terminate the program
exit:
    ecall   3

# The DATA Section
    .data
    # if you don't provide a starting location, the DATA Section
    # starts immediately after the TEXT Section.
    .org    0x100

    # Some data definitions
str:   
    .asciiz "hello world!"
A:
    .byte   50
B:
    .word   0x23A0, 500, 30000
C:
    .space  200

To invoke the assembler, use:

./z16asm [-v] [-d] [-o <binary_file>] <sourcefile>

• -d : Output debugging messages as it runs
• -v : Dump the symol table as well as the Section sizes.
• -o : Specify the binary file name (optional)

In addition to the binary file, the assembler outputs the list file (.lst) that combines the source and machine codes.

Contribution

Please refer to the contribution guide here.