1 # Registers
2 
3 16 bit
4 8 GPRs (zero=0, sp=6, lr=7)
5 1 PC
6 
7 # Instructions
8 
9 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
10 | rep                 | instr               | func                          | F | descr                                      |
11 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
12 | 0000 0RRR 0RRR 0RRR | add rd, rs, rt      | rd = rs + rt                  | R | Addition                                   |
13 | 0000 0RRR 0RRR 1RRR | sub rd, rs, rt      | rd = rs - rt                  | R | Subtraction                                |
14 | 0000 0RRR 1RRR 0RRR | and rd, rs, rt      | rd = rs & rt                  | R | Logical AND                                |
15 | 0000 0RRR 1RRR 1RRR | or rd, rs, rt       | rd = rs or rt                 | R | Logical OR                                 |
16 | 0000 1RRR 0RRR 0RRR | lsh rd, rs, rt      | rd = lshift(rs, rt)           | R | Logical bi-directional shift               |
17 | 0000 1RRR 0RRR 1RRR | xor rd, rs, rt      | rd = rs ^ rt                  | R | Logical XOR                                |
18 | 0000 1RRR 1RRR 0RRR | slt rd, rs, rt      | rd = rs < rt  (signed)        | R | Set if less than (signed)                  |
19 | 0000 1RRR 1RRR 1RRR | sltu rd, rs, rt     | rd = rs < rt (unsigned)       | R | Set if less than (unsigned)                |
20 | 0001 0RRR 0RRR 0RRR | jalr rd, rt         | rd = pc; pc = rt              | R | Jump to Register and Link                  |
21 | 0001 0RRR 1RRR 0RRR | ud                  |                               | R | (undefined)                                |
22 | 0001 1RRR 0RRR 0RRR | ud                  |                               | R |                                            |
23 | 0001 1RRR 1RRR 0RRR | ud                  |                               | R |                                            |
24 | 0001 xRRR xRRR 1RRR | ud                  |                               | R |                                            |
25 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
26 | 0010 0RRR IIII IIII | addi reg, simm8     | rd = rd + simm8               | I | Add w/ immediate                           |
27 | 0010 1RRR IIII IIII | lshi reg, simm8     | rd = lshift(rd, simm8)        | I | Logical bi-directional shift w/ immediate  |
28 | 0011 0RRR IIII IIII | lui rd, imm8        | rd = (imm8 << 8)              | I | Load upper-immediate                       |
29 | 0011 1RRR IIII IIII | li rd, imm8         | rd = imm8                     | I | Load lower-immediate                       |
30 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
31 | 0100 IRRR IIII IRRR | lw rd, [rt, simm6]  | rd = *(rt + simm6)            | L | Load word from memory                      |
32 | 0101 IRRR IIII IRRR | lbu rd, [rt, simm6] | rd = *(rt + simm6)            | L | Load unsigned byte from memory (TODO)      |
33 | 0110 IRRR IIII IRRR | lb rd, [rt, simm6]  | rd = *(rt + simm6)            | L | Load sign-extended byte from memory (TODO) |
34 | 0111 IRRR IIII IRRR | ud                  |                               | L |                                            |
35 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
36 | 1000 IIII IRRR IRRR | sw rs, [rt, simm6]  | *(rt + simm6 * 2) = rs        | S | Store word to memory                       |
37 | 1001 IIII IRRR IRRR | sb rs, [rt, simm6]  | *(rt + simm6) = rs            | S | Store byte to memory (TODO)                |
38 | 1010 IIII IRRR IRRR | beq rs, rt, simm6   | if (rs == rt) pc += simm6 * 2 | S | PC-relative branch if equal                |
39 | 1011 IIII IRRR IRRR | bne rs, rt, simm6   | if (rs != rt) pc += simm6 * 2 | S | PC-relative branch if not equal            |
40 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
41 | 110x xxxx xxxx xxxx | ud                  |                               | ? |                                            |
42 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
43 | 1110 IIII IIII IIII | jal simm12          | lr = pc; pc += simm12 * 2     | J | PC-relative Jump and Link                  |
44 | 1111 IIII IIII IIII | j simm12            | pc += simm12 * 2              | J | PC-relative Jump                           |
45 |---------------------|---------------------|-------------------------------|---|--------------------------------------------|
46 
47 ```c
48 uint16_t lshift(uint16_t a, int16_t b) {
49 	if (b < 0) {
50 		return a >> -b;
51 	} else {
52 		return a << b;
53 	}
54 }
55 ```