diff --git a/homework4/src/homework4.h b/homework4/src/homework4.h
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+++ b/homework4/src/homework4.h
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+// homework4.h
+/*
+ * this header file defines defines:
+ *  the BUS pin numbers for the LEDs, 7SEGs, and 38 transistors.
+ *  the type for the 38 transistor array.
+ */
+
+#include <REG52.H>
+
+// variables:
+// pin addresses for the 38 transistor
+sbit ADDR0 = P1 ^ 0;
+sbit ADDR1 = P1 ^ 1;
+sbit ADDR2 = P1 ^ 2;
+sbit ADDR3 = P1 ^ 3;
+sbit ENLED = P1 ^ 4;
+// BUS pin addresses for the LEDs and 7SEGs:
+sbit BUS1A  = P2 ^ 0;
+sbit BUS2B  = P2 ^ 1;
+sbit BUS3C  = P2 ^ 2;
+sbit BUS4D  = P2 ^ 3;
+sbit BUS5E  = P2 ^ 4;
+sbit BUS6F  = P2 ^ 5;
+sbit BUS7G  = P2 ^ 6;
+sbit BUS8DP = P2 ^ 7;
+
+// functions:
+// 38 transistor type setters:
+void set_38_2LED()
+{
+    // e.g. set Y3 to 1, others to 0:
+    ADDR0 = 0;
+    ADDR1 = 1;
+    ADDR2 = 1;
+    ADDR3 = 1;
+    ENLED = 0;
+}
+void set_38_2SEG()
+{
+    // e.g. set Y4 to 1, others to 0:
+    ADDR0 = 1;
+    ADDR1 = 0;
+    ADDR2 = 0;
+    ADDR3 = 1;
+    ENLED = 0;
+}
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