diff --git a/config/regions.tsv b/config/regions.tsv
index a8afd2c..30bfc52 100644
--- a/config/regions.tsv
+++ b/config/regions.tsv
@@ -157,6 +157,7 @@
 0x800579A0	0x800579F0	src/func_800579A0.c
 0x800579F0	0x80057A40	src/func_800579F0.c
 0x80057AE0	0x80057B30	src/func_80057AE0.c
+0x80057B84	0x80057BE8	src/func_80057B84.c
 0x80057C30	0x80057C80	src/func_80057C30.c
 0x80057DFC	0x80057E04	src/func_80057DFC.c
 0x80058230	0x80058288	src/func_80058230.c
diff --git a/src/func_80057B84.c b/src/func_80057B84.c
new file mode 100644
index 0000000..878c74c
--- /dev/null
+++ b/src/func_80057B84.c
@@ -0,0 +1,57 @@
+/* func_80057B84 — 0x80057B84..0x80057BE8 (100 bytes).
+ *
+ * Fourth member of the 0x80057xxx argument-block family, and the one that differs
+ * structurally from its three matched siblings.
+ *
+ * Original words:
+ *   27BDFFD0  addiu sp,sp,-48
+ *   30A300FF  andi  v1,a1,0xff
+ *   0003102B  sltu  v0,zero,v1
+ *   00021023  negu  v0,v0
+ *   93A80040  lbu   t0,64(sp)        the FIFTH incoming parameter
+ *   00C24824  and   t1,a2,v0         t1 = a2 & mask   (a2 or 0)
+ *   10600003  beqz  v1,0x80057BAC
+ *   AFBF0028  _sw   ra,40(sp)        (delay slot)
+ *   08015EEC  j     0x80057BB0
+ *   AFA70010  _sw   a3,16(sp)        (delay slot) outgoing arg 5 = a3
+ *   AFA00010  sw    zero,16(sp)      <- 0x80057BAC, outgoing arg 5 = 0
+ *   30A200FF  andi  v0,a1,0xff
+ *   24050012  li    a1,18
+ *   2406FFFF  li    a2,-1
+ *   01203821  move  a3,t1
+ *   AFA00014  sw    zero,20(sp)      outgoing arg 6 = 0
+ *   AFA00018  sw    zero,24(sp)      outgoing arg 7 = 0
+ *   AFA0001C  sw    zero,28(sp)      outgoing arg 8 = 0
+ *   AFA20020  sw    v0,32(sp)        outgoing arg 9 = narrowed a1
+ *   0C015D99  jal   0x80057664
+ *   AFA80024  _sw   t0,36(sp)        (delay slot) outgoing arg 10 = the fifth parameter
+ *
+ * **THE BRANCH IS THE TELL, AND IT DISTINGUISHES THIS ROW FROM ITS SIBLINGS.** The three
+ * matched members select between `a0` and zero, which cc1 compiles **branchlessly** with
+ * `sltu`+`negu`+`and`. Here **two different values** are selected — outgoing arg 4 is `a2`
+ * or zero and outgoing arg 5 is `a3` or zero — so the same mask is built for arg 4 while
+ * arg 5 needs a real `beqz`/`j` pair. So the family contains both spellings, and which one a
+ * row uses is decided by *how many distinct values are selected*, not by the author's style:
+ * one value → branchless, two values → one branch. That is a cheap discriminator for the
+ * remaining rows in this family.
+ *
+ * The mask is still built with `sltu`+`negu` (the 0-or-−1 trick) and `negu` appears once
+ * more, consistent with the toolchain evidence this family has been accumulating.
+ *
+ * The callee's ten arguments are: the routine's `a0`, the selector 18, −1, `a2 or 0`,
+ * `a3 or 0`, three zeros, then the narrowed `a1` and the fifth parameter on the stack.
+ *
+ * LIMITS: the selector 18, the mask 0xff and the frame layout are read from the bytes. The
+ * fifth parameter is `unsigned char` by the single `lbu`. As in the siblings, the outgoing
+ * argument area is directly visible — six words at 16–36(sp) with `ra` at 40 — so the
+ * ten-argument model is checkable by arithmetic rather than inferred.
+ */
+
+void func_80057664(int a0, int a1, int a2, int a3, int a4, int a5, int a6, int a7,
+                   int a8, int a9);
+
+void func_80057B84(int a0, int a1, int a2, int a3, unsigned char a4)
+{
+    func_80057664(a0, 18, -1, (a1 & 0xff) ? a2 : 0, (a1 & 0xff) ? a3 : 0, 0, 0, 0,
+                  a1 & 0xff, a4);
+}