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/io.inc65

@@ -1 +1 @@
-BB_SPI_BASE = $7ff0
+SEVEN_SEG  = $7ff0

sw/main.c

@@ -1,11 +1,12 @@
-#define SEVEN_SEG 0x7ff0
-
 #include <stdint.h>
 
-int main() {
-    uint16_t* seven_seg;
-    seven_seg = (uint16_t*)SEVEN_SEG;
+#include "sevenseg.h"
 
-    *seven_seg = 0xbabe;
+int main() {
+    //hex_enable(0xff);
+    hex_set_8(0xb5, 0);
+    hex_set_8(0x00, 1);
+    hex_set_8(0xb0, 2);
+    while(1);
     return 0;
 }

sw/sevenseg.h

@@ -0,0 +1,12 @@
+#ifndef _SEVEN_SEG
+#define _SEVEN_SEG
+
+#include <stdint.h>
+
+uint8_t hex_set_8(uint8_t val, uint8_t idx);
+uint8_t hex_set_16(uint16_t val);
+uint8_t hex_set_24(uint32_t val);
+
+void hex_enable(uint8_t mask);
+
+#endif

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