Add software interface for reading SD card data

Adds functions to wait for data to be read, and to read data form the sd
card controller.

sw/io.inc65

@@ -13,5 +13,6 @@ MM_DATA		= $7fe0
 
 SD_ARG      = $7ff8
 SD_CMD      = $7ffc
+SD_DATA     = $7ffd
 
 IRQ_STATUS  = $7fff

sw/main.c

@@ -87,6 +87,16 @@ int main() {
 	sd_card_resp(&resp);
 	cprintf("CMD17: %lx\n", resp);
 
+
+	while(sw_read());
+
+	sd_card_wait_for_data();
+
+	cprintf("Read data: \n");
+	for (i = 0; i < 512; i++){
+		cprintf("%c", sd_card_read_byte());
+	}
+
 	while (1) {
 
 		sw = sw_read();

sw/sd_card.h

@@ -6,5 +6,7 @@
 void sd_card_command(uint32_t arg, uint8_t cmd);
 
 void sd_card_resp(uint32_t* resp);
+uint8_t sd_card_read_byte();
+void sd_card_wait_for_data();
 
 #endif

sw/sd_card.s

@@ -4,6 +4,8 @@
 
 .export _sd_card_command
 .export _sd_card_resp
+.export _sd_card_read_byte
+.export _sd_card_wait_for_data
 
 .autoimport on
 
@@ -33,6 +35,7 @@ _sd_card_resp:
         sta ptr1        ; store pointer
         stx ptr1+1
 @1:     lda SD_CMD      ; wait for status flag
+        and #$01
         beq @1
         lda SD_ARG
         ldy #$0
@@ -48,3 +51,16 @@ _sd_card_resp:
         sta (ptr1),y
         ply
         rts
+
+_sd_card_read_byte:
+        lda SD_DATA
+        ldx #$00
+        rts
+
+_sd_card_wait_for_data:
+        pha
+@1:     lda SD_CMD      ; wait for status flag
+        and #$02
+        beq @1
+        pla
+        rts