here is a start on test-suite/tests/mmap.test

+;;;; mmap.test --- test suite for Guile's mmap functions  -*- scheme -*-
+;;;;
+
+(define-module (test-mmap)
+  #:use-module (test-suite lib))
+
+(use-modules (rnrs bytevectors))
+
+(with-test-prefix "mmap"
+
+  (pass-if "basics"
+    (let* ((siz #x10000)
+          (reg (mmap 0 siz)))
+      (and (eqv? (bytevector-length reg) siz)
+          (begin (bytevector-u8-set! reg 0 99)
+                 (eqv? (bytevector-u8-ref reg 0) 99))
+          (begin (bytevector-u8-set! reg (1- siz) 98)
+                 (eqv? (bytevector-u8-ref reg (1- siz)) 98))
+          #t)))
+
+  )
+
+;;;; --- last line ---