diff --git a/gcode_process.c b/gcode_process.c
index 25910cf..9b809c8 100644
--- a/gcode_process.c
+++ b/gcode_process.c
@@ -35,6 +35,12 @@ uint8_t tool;
 /// the tool to be changed when we get an M6
 uint8_t next_tool;
 
+/** Bitfield for available sources of G-code.
+
+  A typical source is the SD card or canned G-code. Serial is currently never
+  turned off.
+*/
+enum gcode_source gcode_sources = GCODE_SOURCE_SERIAL;
 
 /************************************************************************//**
 
@@ -406,6 +412,23 @@ void process_gcode_command() {
         //? to the next M23.
         sd_open(gcode_str_buf);
         break;
+
+      case 24:
+        //? --- M24: start/resume SD print. ---
+        //?
+        //? This makes the SD card available as a G-code source. File is the
+        //? one selected with M23.
+        gcode_sources |= GCODE_SOURCE_SD;
+        break;
+
+      case 25:
+        //? --- M25: pause SD print. ---
+        //?
+        //? This removes the SD card from the bitfield of available G-code
+        //? sources. The file is kept open. The position inside the file
+        //? is kept as well, to allow resuming.
+        gcode_sources &= ! GCODE_SOURCE_SD;
+        break;
       #endif /* SD */
 
 			case 82:
diff --git a/gcode_process.h b/gcode_process.h
index 4086b84..21a8b82 100644
--- a/gcode_process.h
+++ b/gcode_process.h
@@ -3,6 +3,14 @@
 
 #include	"gcode_parse.h"
 
+enum gcode_source {
+  GCODE_SOURCE_SERIAL  = 0b00000001,
+  GCODE_SOURCE_SD      = 0b00000010,
+};
+
+
+extern enum gcode_source gcode_sources;
+
 // the current tool
 extern uint8_t tool;
 // the tool to be changed when we get an M6
diff --git a/mendel.c b/mendel.c
index 23cbf76..458050c 100644
--- a/mendel.c
+++ b/mendel.c
@@ -36,6 +36,7 @@
 #include	"dda_queue.h"
 #include	"dda.h"
 #include	"gcode_parse.h"
+#include "gcode_process.h"
 #include	"timer.h"
 #include	"temp.h"
 #include	"sermsg.h"
@@ -270,11 +271,33 @@ int main (void)
 	{
 		// if queue is full, no point in reading chars- host will just have to wait
     if (queue_full() == 0) {
+      uint8_t c;
+
       if (serial_rxchars() != 0) {
-        uint8_t c = serial_popchar();
+        c = serial_popchar();
         gcode_parse_char(c);
       }
 
+      #ifdef SD
+        #include "delay.h"
+
+        if (gcode_sources & GCODE_SOURCE_SD) {
+          c = sd_read_char();
+          /* Demo code here again, just write the character to the serial line,
+             leading to the file written as-is to there. This may help
+             demonstrating correctness of the implementation. */
+          if (c != 0) {
+            serial_writechar(c);
+            delay_us(1000);
+          }
+          else {
+            gcode_sources &= ! GCODE_SOURCE_SD;
+            // There is no pf_close(), subsequent reads will stick at EOF
+            // and return zeros.
+          }
+        }
+      #endif
+
       #ifdef CANNED_CYCLE
         /**
           WARNING!
diff --git a/sd.c b/sd.c
index 6fe93d1..697b88b 100644
--- a/sd.c
+++ b/sd.c
@@ -10,10 +10,15 @@
 #include "serial.h"
 #include "sersendf.h"
 
+#define SD_BUFFER_SIZE 16
+
 
 static FATFS sdfile;
 static FRESULT result;
 
+static uint8_t sd_buffer[SD_BUFFER_SIZE];
+static uint8_t sd_buffer_ptr = SD_BUFFER_SIZE;
+
 /** Initialize SPI for SD card reading.
 */
 void sd_init(void) {
@@ -78,14 +83,43 @@ void sd_list(const char* path) {
 */
 void sd_open(const char* filename) {
   result = pf_open(filename);
-  if (result == FR_OK) {
-    // To demonstrate this works, open the file and show its size.
-    // Actually, the file name isn't stored, so we can't read it back :-)
-    sersendf_P(PSTR("Successfully opened file with %lu bytes."), sdfile.fsize);
-  }
-  else {
+  if (result != FR_OK) {
     sersendf_P(PSTR("E: failed to open file. (%su)"), result);
   }
 }
 
+/** Read a character from a file.
+
+  \return The character read, or zero if there is no such character (e.g. EOF).
+
+  In principle it'd be possible to read the file character by character.
+  However, Before too long this will cause the printer to read G-code from this file
+  until done or until stopped by G-code coming in over the serial line.
+*/
+uint8_t sd_read_char(void) {
+  UINT read;
+  uint8_t this_char;
+
+  if (sd_buffer_ptr == SD_BUFFER_SIZE) {
+    result = pf_read(sd_buffer, SD_BUFFER_SIZE, &read);
+    if (result != FR_OK) {
+      sersendf_P(PSTR("E: failed to read from file. (%su)"), result);
+      return 0;
+    }
+    if (read < SD_BUFFER_SIZE) {
+      sd_buffer[read] = 0;           // A zero marks EOF.
+    }
+
+    sd_buffer_ptr = 0;
+  }
+
+  this_char = sd_buffer[sd_buffer_ptr];
+  if (this_char == 0)
+    sd_buffer_ptr = SD_BUFFER_SIZE;  // Start over, perhaps with next file.
+  else
+    sd_buffer_ptr++;
+
+  return this_char;
+}
+
 #endif /* SD */
diff --git a/sd.h b/sd.h
index 7252add..b7e58a4 100644
--- a/sd.h
+++ b/sd.h
@@ -25,6 +25,8 @@ void sd_list(const char* path);
 
 void sd_open(const char* filename);
 
+uint8_t sd_read_char(void);
+
 #endif /* SD_CARD_SELECT_PIN */
 
 #endif /* _SD_H */