diff --git a/hw/xbox/dsp/dsp_cpu.c b/hw/xbox/dsp/dsp_cpu.c
index 474e38aa04..24eaeb8d6b 100644
--- a/hw/xbox/dsp/dsp_cpu.c
+++ b/hw/xbox/dsp/dsp_cpu.c
@@ -228,7 +228,7 @@ static const OpcodeEntry nonparallel_opcodes[] = {
     { "00001100000111111111gggd", "cmpu S1, S2", dis_cmpu, emu_cmpu },
     { "000000000000001000000000", "debug", NULL, NULL },
     { "00000000000000110000CCCC", "debugcc", NULL, NULL },
-    { "00000000000000000000101d", "dec D", NULL, NULL, /*dis_dec, emu_dec*/ },
+    { "00000000000000000000101d", "dec D", NULL /*dis_dec*/, emu_dec },
     { "000000011000000001JJd000", "div S, D", dis_div, emu_div },
     { "000000010010010s1sdkQQQQ", "dmac S1, S2, D", NULL, NULL },
     { "0000011001MMMRRR0S000000", "do [X or Y]:ea, expr", dis_do_ea, emu_do_ea, match_MMMRRR },
diff --git a/hw/xbox/dsp/dsp_emu.inl b/hw/xbox/dsp/dsp_emu.inl
index 8da806048b..b18259469d 100644
--- a/hw/xbox/dsp/dsp_emu.inl
+++ b/hw/xbox/dsp/dsp_emu.inl
@@ -6605,6 +6605,44 @@ static void emu_cmpu(dsp_core_t* dsp)
     dsp->registers[DSP_REG_SR] |= (dest[0]>>4) & 0x8;
 }
 
+static void emu_dec(dsp_core_t* dsp)
+{
+    uint32_t destreg, source[3], dest[3];
+    uint16_t newsr;
+
+    destreg = DSP_REG_A + (dsp->cur_inst & 1);
+    if (destreg == DSP_REG_A) {
+        dest[0] = dsp->registers[DSP_REG_A2];
+        dest[1] = dsp->registers[DSP_REG_A1];
+        dest[2] = dsp->registers[DSP_REG_A0];
+    } else {
+        dest[0] = dsp->registers[DSP_REG_B2];
+        dest[1] = dsp->registers[DSP_REG_B1];
+        dest[2] = dsp->registers[DSP_REG_B0];
+    }
+
+    source[2] = 1;
+    source[1] = 0;
+    source[0] = 0;
+
+    newsr = dsp_sub56(source, dest);
+
+    if (destreg == DSP_REG_A) {
+        dsp->registers[DSP_REG_A2] = dest[0];
+        dsp->registers[DSP_REG_A1] = dest[1];
+        dsp->registers[DSP_REG_A0] = dest[2];
+    } else {
+        dsp->registers[DSP_REG_B2] = dest[0];
+        dsp->registers[DSP_REG_B1] = dest[1];
+        dsp->registers[DSP_REG_B0] = dest[2];
+    }
+
+    emu_ccr_update_e_u_n_z(dsp, dest[0], dest[1], dest[2]);
+
+    dsp->registers[DSP_REG_SR] &= BITMASK(16)-((1<<DSP_SR_V)|(1<<DSP_SR_C));
+    dsp->registers[DSP_REG_SR] |= newsr;
+}
+
 static void emu_div(dsp_core_t* dsp)
 {
     uint32_t srcreg, destreg, source[3], dest[3];