diff --git a/.gitignore b/.gitignore
index 4c07e34..3ff52c5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
 k.exe
 *.o
 arch_i.h
+
diff --git a/INSTRS b/INSTRS
new file mode 100644
index 0000000..0f9f469
--- /dev/null
+++ b/INSTRS
@@ -0,0 +1,26 @@
+# The OS/K Team licenses this file to you under the MIT license.
+# See the LICENSE file in the project root for more information.
+
+ADD r ri
+    %0 = %0 + %1
+
+SUB r ri
+    %0 = %0 + %1
+
+MUL ri
+    RDX = hi(%0 * %1)
+    RAX = lo(%0 * %1)
+
+DIV ri
+    RDX = RAX % %0
+    RAX = RAX / %0
+
+INC r
+    %0 = %0 + 1
+
+DEC r
+    %0 = %0 - 1
+
+MOV ri ri
+    %0 = %1
+
diff --git a/Makefile b/Makefile
index 4c525ad..f2a2016 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
-# The OS/K Team licences this file to you under the MIT license.
-# See the LICENCE file in the project root for more information.
+# The OS/K Team licenses this file to you under the MIT license.
+# See the LICENSE file in the project root for more information.
 
 all: k.exe
 
diff --git a/instrs.c b/instrs.c
new file mode 100644
index 0000000..f4cce91
--- /dev/null
+++ b/instrs.c
@@ -0,0 +1,141 @@
+// The OS/K Team licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#include "arch.h"
+
+void i_add(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG) {
+        _except(ctx, E_ILL, "ADD into IMM");
+    }
+    
+    if (p1->mem || p2->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+
+    ctx->r[p1->val].val += p2->type >= A_IMM16 ? p2->val
+                                           : ctx->r[p2->val].val;
+}
+
+void i_sub(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG) {
+        _except(ctx, E_ILL, "SUB into IMM");
+    }
+    
+    if (p1->mem || p2->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+    
+    ctx->r[p1->val].val -= p2->type >= A_IMM16 ? p2->val
+                                          : ctx->r[p2->val].val;
+}
+
+void i_mul(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+    
+    // Adapted from www.codeproject.com/Tips/618570/UInt-Multiplication-Squaring
+
+    ulong u = p1->type >= A_IMM16 ? p1->val : ctx->r[p1->val].val;
+    ulong v = ctx->r[RAX].val;
+    ulong u1 = u & 0xffffffff;
+    ulong v1 = v & 0xffffffff;
+    ulong t = (u1 * v1);
+    ulong w3 = (t & 0xffffffff);
+    ulong k = (t >> 32);
+    
+    u >>= 32;
+    t = (u * v1) + k;
+    k = (t & 0xffffffff);
+    ulong w1 = (t >> 32);
+    
+    v >>= 32;
+    t = (u1 * v) + k;
+    k = (t >> 32);
+    
+    ctx->r[RDX].val = (u * v) + w1 + k;
+    ctx->r[RAX].val = (t << 32) + w3;
+}
+
+void i_div(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+
+    ulong val = p1->type >= A_IMM16 ? p1->val : ctx->r[p1->val].val;
+
+    ctx->r[RDX].val = ctx->r[RAX].val % val;
+    ctx->r[RAX].val = ctx->r[RAX].val / val;
+}
+
+void i_inc(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG) {
+        _except(ctx, E_ILL, "INC on an IMM");
+    }
+
+    if (p1->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+    
+    ctx->r[p1->val].val++;
+}
+
+void i_dec(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG) {
+        _except(ctx, E_ILL, "DEC on an IMM");
+    }
+
+    if (p1->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+    
+    ctx->r[p1->val].val--;
+}
+
+void i_mov(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG) {
+        _except(ctx, E_ILL, "MOV into IMM");
+    }
+    
+    if (p1->mem || p2->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+
+    ctx->r[p1->val].val = p2->type >= A_IMM16 ? p2->val
+                                        : ctx->r[p2->val].val;
+}
+
+void i_xchg(ctx_t *ctx, acc_t *p1, acc_t *p2)
+{
+    if (p1->type != A_REG || p2->type != A_REG) {
+        _except(ctx, E_ILL, "XCHG of IMM(s)");
+    }
+    
+    if (p1->mem || p2->mem) {
+        _except(ctx, E_IMP, "Memory access");
+    }
+
+    ulong temp = ctx->r[p1->val].val;
+    ctx->r[p1->val].val = ctx->r[p2->val].val;
+    ctx->r[p2->val].val = temp;
+}
+
+instr_t arch_i[NINSTRS] =
+{
+    [I_ADD] = { "ADD", 2, i_add },
+    [I_SUB] = { "SUB", 2, i_sub },
+    [I_MUL] = { "MUL", 1, i_mul },
+    [I_DIV] = { "DIV", 1, i_div },
+    [I_INC] = { "INC", 1, i_inc },
+    [I_DEC] = { "DEC", 1, i_dec },
+    [I_MOV] = { "MOV", 2, i_mov },
+    [I_XCHG] = { "XCHG", 2, i_xchg },
+};
+
diff --git a/karch/arch.c b/karch/arch.c
index 1f10b0c..963b1c0 100644
--- a/karch/arch.c
+++ b/karch/arch.c
@@ -1,5 +1,5 @@
-// The OS/K Team licences this file to you under the MIT license.
-// See the LICENCE file in the project root for more information.
+// The OS/K Team licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
 
 #include "arch.h"
 
diff --git a/karch/arch.h b/karch/arch.h
index e77bfb6..60848c7 100644
--- a/karch/arch.h
+++ b/karch/arch.h
@@ -1,5 +1,5 @@
-// The OS/K Team licences this file to you under the MIT license.
-// See the LICENCE file in the project root for more information.
+// The OS/K Team licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
 
 #include <assert.h>
 #include <stdio.h>
diff --git a/karch/regs.c b/karch/regs.c
index 9c5082f..2aa379e 100644
--- a/karch/regs.c
+++ b/karch/regs.c
@@ -1,5 +1,5 @@
-// The OS/K Team licences this file to you under the MIT license.
-// See the LICENCE file in the project root for more information.
+// The OS/K Team licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
 
 #include "arch.h"