Add SD card software interface

Adds a function to send a command to the sd card.

sw/io.inc65

@@ -11,4 +11,7 @@ SW          = LED
 MM_CTRL		= $7ff7
 MM_DATA		= $7fe0
 
+SD_ARG      = $7ff8
+SD_CMD      = $7ffc
+
 IRQ_STATUS  = $7fff

sw/main.c

@@ -4,6 +4,7 @@
 #include "board_io.h"
 #include "uart.h"
 #include "mapper.h"
+#include "sd_card.h"
 
 int main() {
     int i;
@@ -46,6 +47,9 @@ int main() {
 	cprintf("Reading from 0x4000: %x\n", *(unsigned int*)(0x4000));
 	cprintf("Reading from 0x5000: %x\n", *(unsigned int*)(0x5000));
 
+	sd_card_command(0, 0);
+	sd_card_command(0x000001aa, 8);
+
     while (1) {
 
         sw = sw_read();

sw/sd_card.h

@@ -0,0 +1,8 @@
+#ifndef _SD_CARD_H
+#define _SD_CARD_H
+
+#include <stdint.h>
+
+void sd_card_command(uint32_t arg, uint8_t cmd);
+
+#endif

sw/sd_card.s

@@ -0,0 +1,27 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _sd_card_command
+
+.autoimport on
+
+.code
+
+; Send sd card command.
+; command is in A register, the args are on the stack
+; I think the order is high byte first?
+_sd_card_command:
+    pha
+
+    jsr popeax
+    sta SD_ARG
+    stx SD_ARG+1
+    lda sreg
+    sta SD_ARG+2
+    lda sreg+1
+    sta SD_ARG+3
+
+    pla
+    sta SD_CMD
+    rts