Add driver for hex digits

This only includes the method to set the pairs of displays individually.
The functions to write a single 16 or 32 bit value have not been
implemented yet, nor has the mask function.

sw/sevenseg.s

@@ -0,0 +1,39 @@
+.include "io.inc65"
+
+.export _hex_set_8
+.export _hex_set_16
+.export _hex_set_24
+.export _hex_enable
+
+.autoimport	on
+
+.code
+
+; @in A: idx Stack[0]: val
+; @out A: 0 for success, 1 for failure.
+; Sets one of the 3 pairs of hex digits.
+_hex_set_8:
+        phx
+        cmp #$3         ; If idx >= 3 then fail
+        bcc @1
+        lda #$1
+        rts
+@1:     tax             ; Move idx into x
+        jsr popa        ; put val into a
+        sta SEVEN_SEG,x ; write to val
+        lda #$0
+        plx
+        rts
+
+_hex_set_16:
+        lda #$1
+        rts
+
+_hex_set_24:
+        lda #$1
+        rts
+
+; @in A: mask
+; Set the mask for seven seg enables
+_hex_enable:
+        rts