rc2014: improve SD card driver docs

blk/600

@@ -1,3 +1,13 @@
 SD Card driver
 
 Load range: 602-616
+
+This driver is designed for a ad-hoc adapter card that acts as a
+SPI relay between the z80 bus and the SD card. That adapter is  
+expected to pull CS low when something is written to SDC_CSLOW,
+high on SDC_CSHIGH and to initiate a SPI exchange when a byte is
+written to SDC_SPI, the result of that exchange being fetched   
+with a read to SDC_SPI.
+
+Through that layer, this driver implements the SDC protocol     
+allowing it to provide BLK@ and BLK!.

blk/604

@@ -13,4 +13,3 @@
     DUP 20 = UNTIL
     DROP 0xff
 ;
-1 12 LOADR+

recipes/rc2014/sdcard/README.md

@@ -86,7 +86,7 @@ Build it and write it to EEPROM.
 The RC2014 emulator includes SDC emulation. You can attach a SD card image to
 it by invoking it with a second argument:
 
-    ../../../emul/hw/rc2014/classic os.bin ../../../emul/blkfs
+    ../../../emul/hw/rc2014/classic os.bin ../../../cvm/blkfs
 
 You will then run with a SD card having the contents from `/blk`.
 

recipes/rc2014/sdcard/cfsin/hello.txt

@@ -1 +0,0 @@
-Hello World!