avr: added routine to count used buffers. fixed a bug.

avr/common/m_fixedbuffers.asm

@@ -108,5 +108,32 @@ l_end:
 
 
 
+; ---------------------------------------------------------------------------
+; @macro m_fixedbuf_count_used
+;
+; @return r16 number of buffers in use
+; @param X pointer to start of buffers
+; @param %0 constant maxBytes per buffer (including statusbyte in front)
+; @param %1 constant maxBuffers
+; @clobbers r16, r17, r18, X
+
+.macro m_fixedbuf_count_used
+  clr r16
+  ldi r17, @1
+l_loop:
+    ld r18, X
+    tst r18
+    breq l_notused
+    inc r16
+l_notused:
+    adiw xh:xl, @0
+    dec r17
+  brne l_loop
+l_end:
+.endmacro
+; @end
+
+
+