Get SD card working in SPI

sw/bootloader/devices/spi.s

@@ -0,0 +1,39 @@
+.include "io.inc65"
+
+.importzp zp, sreg
+
+.export _spi_select, _spi_deselect
+.export _spi_read, _spi_write, _spi_exchange
+
+.autoimport on
+
+.code
+
+; void spi_select(uint8_t id)
+; Select a (the) slave by pulling its CS line down
+; TODO allow active high or active low CS
+; TODO allow more than one slave
+_spi_select:
+    lda #$1         ; Ignore whatever id is, 1 is the only option
+    sta SPI_CTRL
+    rts
+
+; void spi_deslect(uint8_t id)
+; Deslect a slave by pulling its CS line up
+; TODO allow active high or active low CS
+_spi_deselect:
+    stz SPI_CTRL
+    rts
+
+; uint8_t spi_read()
+_spi_read:
+    lda #$0
+; void spi_write(uint8_t data)
+_spi_write:
+; uint8_t spi_exchange(uint8_t data)
+_spi_exchange:
+    sta SPI_OUTPUT
+@1: lda SPI_CTRL
+    bmi @1
+    lda SPI_INPUT
+    rts