diff --git a/day21p1/Makefile b/day21p1/Makefile
new file mode 100644
index 0000000..c711967
--- /dev/null
+++ b/day21p1/Makefile
@@ -0,0 +1,12 @@
+CFLAGS := -Wall -Wextra -std=c17 -D_GNU_SOURCE
+
+LIBS := glib-2.0
+CFLAGS += $(shell pkg-config --cflags $(LIBS))
+LDLIBS := $(shell pkg-config --libs $(LIBS))
+
+.PHONY: all
+all: main
+
+.PHONY: clean
+clean:
+	rm -f main
diff --git a/day21p1/input b/day21p1/input
new file mode 100644
index 0000000..36bea34
--- /dev/null
+++ b/day21p1/input
@@ -0,0 +1,32 @@
+#ip 4
+seti 123 0 2
+bani 2 456 2
+eqri 2 72 2
+addr 2 4 4
+seti 0 0 4
+seti 0 8 2
+bori 2 65536 5
+seti 2238642 0 2
+bani 5 255 3
+addr 2 3 2
+bani 2 16777215 2
+muli 2 65899 2
+bani 2 16777215 2
+gtir 256 5 3
+addr 3 4 4
+addi 4 1 4
+seti 27 3 4
+seti 0 8 3
+addi 3 1 1
+muli 1 256 1
+gtrr 1 5 1
+addr 1 4 4
+addi 4 1 4
+seti 25 4 4
+addi 3 1 3
+seti 17 2 4
+setr 3 9 5
+seti 7 9 4
+eqrr 2 0 3
+addr 3 4 4
+seti 5 0 4
diff --git a/day21p1/input.dec b/day21p1/input.dec
new file mode 100644
index 0000000..33df5c2
--- /dev/null
+++ b/day21p1/input.dec
@@ -0,0 +1,68 @@
+#ip 4
+
+; while (123 & 456 != 72);
+00: seti 123 0 2
+01: bani 2 456 2
+02: eqri 2 72 2
+03: addr 2 4 4
+04: seti 0 0 4
+
+05: seti 0 8 2          ; r2 = 0
+; xref 30
+06: bori 2 65536 5      ; r5 = r2 | 0x10000
+07: seti 2238642 0 2    ; r2 = 2238642
+; xref 27
+08: bani 5 255 3        ; r3 = r5 & 0xff
+09: addr 2 3 2          ; r2 = r2 + r3
+10: bani 2 16777215 2   ; r2 = r2 & 0xffffff
+11: muli 2 65899 2      ; r2 = r2 * 65899
+12: bani 2 16777215 2   ; r2 = r2 & 0xffffff
+; trash = r3
+; keep = r2, r5
+
+13: gtir 256 5 3        ; if (0x100 > r5)
+14: addr 3 4 4          ; then
+15: addi 4 1 4          ; (false) jr +1 (17)
+16: seti 27 3 4         ; (true ) jp 27 (28)
+
+; xref 16 (256 <= r5)
+17: seti 0 8 3          ; r3 = 0
+
+; while ((r3 + 1) * 0x100 <= r5) r3++
+18: addi 3 1 1          ; r1 = r3 + 1
+19: muli 1 256 1        ; r1 = r1 * 0x100
+20: gtrr 1 5 1          ; if (r1 > r5)
+21: addr 1 4 4          ; then
+22: addi 4 1 4          ; ..
+23: seti 25 4 4         ; ..
+24: addi 3 1 3          ; (false) r3 = r3 + 1
+25: seti 17 2 4         ; (false) jp 17 (18)
+
+26: setr 3 9 5          ; (true ) r5 = r3
+27: seti 7 9 4          ; (true ) jp 07 (08)
+
+; xref 16 (256 > r5)
+28: eqrr 2 0 3          ; if (r2 == r0)
+29: addr 3 4 4          ; then
+30: seti 5 0 4          ; (false) jp 05 (06)
+                        ; (true ) exit
+
+; ===== HIGH-LEVEL PSEUDOCODE =====
+
+r2 = 0
+
+do {
+    r5 = r2 | 0x10000
+    r2 = 2238642
+
+    while (1) {
+        // r2 = (r2 + r5) * 65899
+        r2 = (((r2 + (r5 & 0xff)) & 0xffffff) * 65899) & 0xffffff
+
+        if (0x100 > r5) break
+
+        r3 = 0
+        while ((r3 + 1) * 0x100 <= r5) r3++
+        r5 = r3
+    }
+} while (r2 != r0)
diff --git a/day21p1/main.c b/day21p1/main.c
new file mode 100644
index 0000000..2ac228f
--- /dev/null
+++ b/day21p1/main.c
@@ -0,0 +1,210 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+#include <glib.h>
+
+#define REGS 6
+
+typedef int reg_t;
+
+enum instr_id {
+    INSTR_ADDR,
+    INSTR_ADDI,
+    INSTR_MULR,
+    INSTR_MULI,
+    INSTR_BANR,
+    INSTR_BANI,
+    INSTR_BORR,
+    INSTR_BORI,
+    INSTR_SETR,
+    INSTR_SETI,
+    INSTR_GTIR,
+    INSTR_GTRI,
+    INSTR_GTRR,
+    INSTR_EQIR,
+    INSTR_EQRI,
+    INSTR_EQRR,
+    INSTR_ID_COUNT
+};
+
+struct instr {
+    enum instr_id i;
+    int a;
+    int b;
+    int c;
+};
+
+const char *instr_str[] = {
+    "addr",
+    "addi",
+    "mulr",
+    "muli",
+    "banr",
+    "bani",
+    "borr",
+    "bori",
+    "setr",
+    "seti",
+    "gtir",
+    "gtri",
+    "gtrr",
+    "eqir",
+    "eqri",
+    "eqrr"
+};
+
+void instr_interpret(reg_t *r, struct instr *i)
+{
+    switch (i->i) {
+    case INSTR_ADDR:
+    case INSTR_MULR:
+    case INSTR_BANR:
+    case INSTR_BORR:
+    case INSTR_GTRR:
+    case INSTR_EQRR:
+        if (i->b >= REGS) return;
+        // fallthrough
+    case INSTR_ADDI:
+    case INSTR_MULI:
+    case INSTR_BANI:
+    case INSTR_BORI:
+    case INSTR_SETR:
+    case INSTR_GTRI:
+    case INSTR_EQRI:
+        if (i->a >= REGS) return;
+        // fallthrough
+    case INSTR_SETI:
+        if (i->c >= REGS) return;
+        break;
+
+    case INSTR_GTIR:
+    case INSTR_EQIR:
+        if (i->b >= REGS) return;
+        if (i->c >= REGS) return;
+        break;
+
+    default:
+        break;
+    }
+
+    switch (i->i) {
+    case INSTR_ADDR: r[i->c] = r[i->a] + r[i->b]; break;
+    case INSTR_ADDI: r[i->c] = r[i->a] + i->b; break;
+    case INSTR_MULR: r[i->c] = r[i->a] * r[i->b]; break;
+    case INSTR_MULI: r[i->c] = r[i->a] * i->b; break;
+    case INSTR_BANR: r[i->c] = r[i->a] & r[i->b]; break;
+    case INSTR_BANI: r[i->c] = r[i->a] & i->b; break;
+    case INSTR_BORR: r[i->c] = r[i->a] | r[i->b]; break;
+    case INSTR_BORI: r[i->c] = r[i->a] | i->b; break;
+    case INSTR_SETR: r[i->c] = r[i->a]; break;
+    case INSTR_SETI: r[i->c] = i->a; break;
+    case INSTR_GTIR: r[i->c] = (i->a > r[i->b]) ? 1 : 0; break;
+    case INSTR_GTRI: r[i->c] = (r[i->a] > i->b) ? 1 : 0; break;
+    case INSTR_GTRR: r[i->c] = (r[i->a] > r[i->b]) ? 1 : 0; break;
+    case INSTR_EQIR: r[i->c] = (i->a == r[i->b]) ? 1 : 0; break;
+    case INSTR_EQRI: r[i->c] = (r[i->a] == i->b) ? 1 : 0; break;
+    case INSTR_EQRR: r[i->c] = (r[i->a] == r[i->b]) ? 1 : 0; break;
+    default: break;
+    }
+}
+
+struct instr *parse_instructions(const char *fname, int *len, int *ip)
+{
+    FILE *f = fopen(fname, "r");
+    if (!f) {
+        perror(fname);
+        return NULL;
+    }
+
+    GArray *array = g_array_new(FALSE, FALSE, sizeof(struct instr));
+
+    char *line = NULL;
+    size_t line_len = 0;
+    while (getline(&line, &line_len, f) != -1) {
+        char *args = strchr(line, ' ');
+        if (!args) continue;
+        *args++ = '\0';
+
+        if (strcmp(line, "#ip") == 0) {
+            *ip = strtol(args, NULL, 0);
+            continue;
+        }
+
+        struct instr instr = {0};
+
+        for (int i = 0; i < INSTR_ID_COUNT; i++) {
+            if (strcmp(line, instr_str[i]) == 0) {
+                instr.i = i;
+                break;
+            }
+        }
+
+        sscanf(args, "%d %d %d", &instr.a, &instr.b, &instr.c);
+        g_array_append_val(array, instr);
+    }
+
+    free(line);
+    fclose(f);
+
+    *len = array->len;
+    return (struct instr *)g_array_free(array, FALSE);
+}
+
+void print_regs(reg_t *regs, FILE *f)
+{
+    putc('[', f);
+    for (int i = 0; i < REGS - 1; i++) {
+        fprintf(f, "%d, ", regs[i]);
+    }
+    fprintf(f, "%d]", regs[REGS - 1]);
+}
+
+int main(int argc, char *argv[])
+{
+    int debug = 0;
+    if (argc > 1 && strcmp(argv[1], "-debug") == 0) debug = 1;
+
+    int ip = 0;
+    int instructions_len = 0;
+    struct instr *instructions = parse_instructions("input", &instructions_len, &ip);
+    if (!instructions) return 1;
+    if (ip >= REGS) ip = 0;
+
+    reg_t *regs = g_new0(reg_t, REGS);
+    while (regs[ip] >= 0 && regs[ip] < instructions_len) {
+        struct instr *instr = &instructions[regs[ip]];
+
+        if (debug) {
+            fprintf(stderr, "ip=%d ", regs[ip]);
+            print_regs(regs, stderr);
+            fprintf(stderr, " %s %d %d %d ",
+                    instr_str[instr->i], instr->a, instr->b, instr->c);
+        }
+
+        instr_interpret(regs, instr);
+
+        if (debug) {
+            print_regs(regs, stderr);
+            putc('\n', stderr);
+        }
+
+        // Day 21 part 1 solution...
+        if (instr->i == INSTR_EQRR && (instr->a == 0 || instr->b == 0)) {
+            int reg = instr->a;
+            if (reg == 0) reg = instr->b;
+            if (reg != 0 && reg < REGS) {
+                printf("Register compared: %d\n", reg);
+                break;
+            }
+        }
+
+        regs[ip]++;
+    }
+
+    print_regs(regs, stdout);
+    putchar('\n');
+
+    free(regs);
+    free(instructions);
+    return 0;
+}