as: add initial, primitive, but working assembler

as/input.h

@@ -0,0 +1,53 @@
+/*
+ * input.h
+ */
+#ifndef INPUT_H
+#define INPUT_H
+
+/*
+ * A directives, label, or instruction is parsed to a statement, which is
+ * either interpreted immediately or appended to the representation of the
+ * current section.
+ */
+
+enum stmt_tag {
+    /* directives */
+    S_DOT_GLOBL,
+    S_DOT_TEXT,
+    /* non-directives */
+    S_LABEL,
+    S_INSN,
+};
+
+struct stmt {
+    struct stmt *next;
+    enum stmt_tag tag;
+    union {
+	struct {	/* S_DOT_GLOBL, S_LABEL */
+	    const char *name;
+	} symbol;
+	struct {	/* S_INSN */
+	    unsigned int opcode;
+	    unsigned int accumulator;
+	    int at;
+	    unsigned int address;	/* XXX: relocatable expr */
+	    unsigned int indexreg;
+	} insn;
+    } u;
+};
+
+/*
+ * The input unit object is the top-level container for the representation
+ * of the sections, and all other information collected from the input.
+ */
+
+struct iunit {
+    struct {
+	struct stmt *head;
+	struct stmt **tailptr;
+    } text;
+};
+
+int input(const char *progname, char **files, int nrfiles, struct iunit *iunit);
+
+#endif /* INPUT_H */