Add basic spi code

Implements the bit bang spi protocol

sw/io.inc65

@@ -0,0 +1 @@
+BB_SPI_BASE = $7ff0

sw/spi.h

@@ -0,0 +1,8 @@
+#ifndef _SPI_H
+#define _SPI_H
+
+#include <stdint.h>
+
+void spi_write_byte(uint8_t);
+
+#endif

sw/spi.s

@@ -0,0 +1,49 @@
+.include "io.inc65"
+
+.export _spi_write_byte
+
+.importzp	sp, sreg, regsave, regbank
+.importzp	tmp1, tmp2, tmp3, tmp4, ptr1, ptr2, ptr3, ptr4
+
+.code
+
+
+;   Write a single byte to the SPI device
+;   @in A The byte to write 
+
+_spi_write_byte:
+        phx                 ; Save regs
+        phy
+        sta tmp1            ; Save value into tmp1
+        lda #$80
+        tax
+@loop:  bit tmp1            ; Check if high bit set
+        beq @1
+        lda #$04            ; Bit not set.
+        bra @1
+@1:     lda #$00            ; Bit set
+        sta BB_SPI_BASE     ; Write data
+        adc #$01
+        sta BB_SPI_BASE     ; Write clock
+        txa
+        lsr                 ; Select net bit
+        bne @loop           ; Stop when mask is 0
+        ply                 ; Restore regs
+        plx
+        rts                 ; Return
+
+
+
+
+    and #$01            ; Get first bit
+    asl
+    asl
+    sta BB_SPI_BASE     ; write bit without clock
+    adc #$01            
+    sta BB_SPI_BASE     ; write bit with clock
+    tay
+    and #$022
+
+    ply
+    plx
+    rts