Initial Checkin

13 files changed, 790 insertions, 0 deletions

diff --git a/software/demo_gpio/Makefile b/software/demo_gpio/Makefile
new file mode 100644
index 0000000..5f201a2
--- /dev/null
+++ b/software/demo_gpio/Makefile
@@ -0,0 +1,9 @@
+#See LICENSE for license details.
+
+TARGET=demo_gpio
+CFLAGS += -g
+CDEFINES += -DUSE_PLIC -DUSE_M_TIME
+
+SWDIR = ../
+
+include $(SWDIR)/shared/Makefile.shared
diff --git a/software/demo_gpio/demo_gpio.c b/software/demo_gpio/demo_gpio.c
new file mode 100644
index 0000000..3b0b6f5
--- /dev/null
+++ b/software/demo_gpio/demo_gpio.c
@@ -0,0 +1,182 @@
+// See LICENSE for license details.
+
+#include <stdio.h>
+
+#include "shared.h"
+#include "plic.h"
+#include <string.h>
+
+void reset_demo (void);
+
+// Structures for registering different interrupt handlers
+// for different parts of the application.
+typedef void (*function_ptr_t) (void);
+
+void no_interrupt_handler (void) {};
+
+function_ptr_t g_ext_interrupt_handlers[9];
+
+function_ptr_t g_m_timer_interrupt_handler = no_interrupt_handler;
+
+// Instance data for the PLIC.
+
+plic_instance_t g_plic;
+
+// Simple variables for LEDs, buttons, etc.
+volatile unsigned int* g_outputs  = (unsigned int *) (GPIO_BASE_ADDR + GPIO_OUT_OFFSET);
+volatile unsigned int* g_inputs   = (unsigned int *) (GPIO_BASE_ADDR + GPIO_IN_OFFSET);
+volatile unsigned int* g_tristates = (unsigned int *) (GPIO_BASE_ADDR + GPIO_TRI_OFFSET);
+
+
+/*Entry Point for PLIC Interrupt Handler*/
+void handle_m_ext_interrupt(){
+  plic_source int_num  = PLIC_claim_interrupt(&g_plic);
+  if ((int_num >=1 ) && (int_num <= 8)) {
+    g_ext_interrupt_handlers[int_num]();
+  }
+  else {
+    _exit(1 + (uintptr_t) int_num);
+  }
+  PLIC_complete_interrupt(&g_plic, int_num);
+}
+
+
+/*Entry Point for Machine Timer Interrupt Handler*/
+void handle_m_time_interrupt(){
+
+  clear_csr(mie, MIP_MTIP);
+  
+  // Reset the timer for 3s in the future.
+  // This also clears the existing timer interrupt.
+  
+  volatile uint64_t * mtime       = (uint64_t*) MTIME_ADDR;
+  volatile uint64_t * mtimecmp    = (uint64_t*) MTIMECMP_BASE_ADDR;
+  uint64_t now = *mtime;
+  uint64_t then = now + 3 * CLOCK_FREQUENCY / RTC_PRESCALER;
+  *mtimecmp = then;
+ 
+  // read the current value of the LEDS and invert them.
+  uint32_t leds = *g_outputs;
+		   
+  *g_outputs = (~leds) & ((0xF << RED_LEDS_OFFSET)   |
+			  (0xF << GREEN_LEDS_OFFSET) |
+			  (0xF << BLUE_LEDS_OFFSET));
+  
+
+  // Re-enable the timer interrupt.
+  set_csr(mie, MIP_MTIP);
+ 
+}
+
+
+/*Entry Point for Machine Timer Interrupt  Handler*/
+void handle_m_timer_interrupt(){
+  g_m_timer_interrupt_handler();
+}
+
+const char * instructions_msg = " \
+\n\
+                SIFIVE, INC.\n\
+\n\
+         5555555555555555555555555\n\
+        5555                   5555\n\
+       5555                     5555\n\
+      5555                       5555\n\
+     5555       5555555555555555555555\n\
+    5555       555555555555555555555555\n\
+   5555                             5555\n\
+  5555                               5555\n\
+ 5555                                 5555\n\
+5555555555555555555555555555          55555\n\
+ 55555           555555555           55555\n\
+   55555           55555           55555\n\
+     55555           5           55555\n\
+       55555                   55555\n\
+         55555               55555\n\
+           55555           55555\n\
+             55555       55555\n\
+               55555   55555\n\
+                 555555555\n\
+                   55555\n\
+                     5\n\
+\n\
+ SiFive E-Series Coreplex Demo on Arty Board\n \
+ \n\
+ ";
+
+void print_instructions() {
+  
+  write (STDOUT_FILENO, instructions_msg, strlen(instructions_msg));
+  
+}
+
+void button_0_handler(void) {
+
+  // Rainbow LEDs!
+  * g_outputs = (0x9 << RED_LEDS_OFFSET) | (0xA << GREEN_LEDS_OFFSET) | (0xC << BLUE_LEDS_OFFSET);
+
+};
+
+
+void reset_demo (){
+
+    // Disable the machine & timer interrupts until setup is done.
+  
+    clear_csr(mie, MIP_MEIP);
+    clear_csr(mie, MIP_MTIP);
+  
+    g_ext_interrupt_handlers[INT_DEVICE_BUTTON_0] = button_0_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_BUTTON_1] = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_BUTTON_2] = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_BUTTON_3] = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_JA_7]     = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_JA_8]     = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_JA_9]     = no_interrupt_handler;
+    g_ext_interrupt_handlers[INT_DEVICE_JA_10]    = no_interrupt_handler;
+    
+    print_instructions();
+
+    PLIC_enable_interrupt (&g_plic, INT_DEVICE_BUTTON_0);
+       
+    // Set the machine timer to go off in 3 seconds.
+    // The
+    volatile uint64_t * mtime       = (uint64_t*) MTIME_ADDR;
+    volatile uint64_t * mtimecmp    = (uint64_t*) MTIMECMP_BASE_ADDR;
+    uint64_t now = *mtime;
+    uint64_t then = now + 3*CLOCK_FREQUENCY / RTC_PRESCALER;
+    *mtimecmp = then;
+
+    // Enable the Machine-External bit in MIE
+    set_csr(mie, MIP_MEIP);
+
+    // Enable the Machine-Timer bit in MIE
+    set_csr(mie, MIP_MTIP);
+    
+    // Enable interrupts in general.
+    set_csr(mstatus, MSTATUS_MIE);
+
+}
+
+int main(int argc, char **argv)
+{
+  // Set up the GPIOs such that the inputs' tristate are set.
+  // The boot ROM actually does this, but still good.
+  
+  * g_tristates = (0xF << BUTTONS_OFFSET) | (0xF << SWITCHES_OFFSET) | (0xF << JA_IN_OFFSET);
+  
+  * g_outputs = (0xF << RED_LEDS_OFFSET);
+
+
+  /**************************************************************************
+   * Set up the PLIC
+   * 
+   *************************************************************************/
+  PLIC_init(&g_plic, PLIC_BASE_ADDR, PLIC_NUM_SOURCES, PLIC_NUM_PRIORITIES);
+
+  reset_demo();
+
+  while (1);
+
+  return 0;
+
+}
diff --git a/software/shared/Makefile.shared b/software/shared/Makefile.shared
new file mode 100644
index 0000000..5817826
--- /dev/null
+++ b/software/shared/Makefile.shared
@@ -0,0 +1,47 @@
+# See LICENSE for license details.
+
+SHARED_DIR = $(SWDIR)/shared
+
+ASM_SRCS += $(SHARED_DIR)/entry.S
+
+C_SRCS += $(SHARED_DIR)/init.c
+C_SRCS += $(SHARED_DIR)/syscall.c
+C_SRCS += $(SHARED_DIR)/drivers_sifive/plic.c
+
+LINKER_SCRIPT := $(SHARED_DIR)/link.lds
+HEADERS += $(SHARED_DIR)/*.h
+
+C_SRCS += $(TARGET).c
+
+INCLUDES += -I .
+INCLUDES += -I $(SHARED_DIR)
+INCLUDES += -I $(SWDIR)/../riscv-tests/env
+INCLUDES += -I$(SHARED_DIR)/drivers_sifive/
+
+CC = $(SWDIR)/../toolchain/bin/riscv32-unknown-elf-gcc
+
+CFLAGS += $(CDEFINES)
+
+LDFLAGS := -T $(LINKER_SCRIPT) -nostdlib -nostartfiles -lc -lgcc
+LDFLAGS += -L$(SHARED_DIR)
+LDFLAGS += -L$(SHARED_DIR)/drivers
+LDFLAGS += -L$(SHARED_DIR)/hal
+
+ASM_OBJS := $(patsubst %.S,%.o,$(ASM_SRCS))
+C_OBJS := $(patsubst %.c,%.o,$(C_SRCS))
+
+$(TARGET): $(ASM_OBJS) $(C_OBJS) $(LINKER_SCRIPT)
+	$(CC) $(CFLAGS) $(INCLUDES) $(C_OBJS) $(ASM_OBJS) -o $@ $(LDFLAGS)
+
+$(ASM_OBJS): %.o: %.S $(HEADERS)
+	$(CC) $(CFLAGS) $(INCLUDES) -c -o $@ $<
+
+$(C_OBJS): %.o: %.c $(HEADERS)
+	$(CC) $(CFLAGS) $(INCLUDES) -c -o $@ $<
+
+all: default
+
+clean:
+	rm -f $(TARGET) *.o $(SHARED_DIR)/*.o $(SHARED_DIR)/drivers/*/*.o $(SHARED_DIR)/drivers_sifive/*.o
+
+.PHONY: clean all default
diff --git a/software/shared/drivers_sifive/plic.c b/software/shared/drivers_sifive/plic.c
new file mode 100644
index 0000000..ec0c178
--- /dev/null
+++ b/software/shared/drivers_sifive/plic.c
@@ -0,0 +1,123 @@
+// See LICENSE for license details.
+
+#include "plic.h"
+#include "encoding.h"
+#include <string.h>
+
+
+// Note that there are no assertions or bounds checking on these
+// parameter values.
+
+void volatile_memzero(uint8_t * base, unsigned int size){
+
+  volatile uint8_t * ptr;
+  for (ptr = base; ptr < (base + size); ptr++){
+    *ptr = 0;
+  }
+}
+  
+void PLIC_init (
+                plic_instance_t * this_plic,
+                uintptr_t            base_addr,
+                uint32_t num_sources,
+                uint32_t num_priorities
+                ){
+  
+  this_plic->base_addr = base_addr;
+  this_plic->num_sources = num_sources;
+  this_plic->num_priorities = num_priorities;
+  
+  // Disable all interrupts (don't assume that these registers are reset).
+  unsigned long hart_id = read_csr(mhartid);
+  volatile_memzero((uint8_t*) (this_plic->base_addr +
+                               PLIC_ENABLE_OFFSET +
+                               (hart_id << PLIC_ENABLE_SHIFT_PER_TARGET)),
+                   (num_sources + 8) / 8);
+  
+  // Set all priorities to 0 (equal priority -- don't assume that these are reset).
+  volatile_memzero ((uint8_t *)(this_plic->base_addr +
+                                PLIC_PRIORITY_OFFSET),
+                    (num_sources + 1) << PLIC_PRIORITY_SHIFT_PER_SOURCE);
+
+  // Set the threshold to 0.
+  volatile plic_threshold* threshold = (plic_threshold*)
+    (this_plic->base_addr +
+     PLIC_THRESHOLD_OFFSET +
+     (hart_id << PLIC_THRESHOLD_SHIFT_PER_TARGET));
+
+  *threshold = 0;
+  
+}
+
+void PLIC_set_threshold (plic_instance_t * this_plic, plic_threshold threshold){
+
+  unsigned long hart_id = read_csr(mhartid);  
+  volatile plic_threshold* threshold_ptr = (plic_threshold*) (this_plic->base_addr +
+                                                              PLIC_THRESHOLD_OFFSET +
+                                                              (hart_id << PLIC_THRESHOLD_SHIFT_PER_TARGET));
+
+  *threshold_ptr = threshold;
+
+}
+  
+
+void PLIC_enable_interrupt (plic_instance_t * this_plic, plic_source source){
+
+  unsigned long hart_id = read_csr(mhartid);
+  volatile uint8_t * current_ptr = (volatile uint8_t *)(this_plic->base_addr +
+                                                        PLIC_ENABLE_OFFSET +
+                                                        (hart_id << PLIC_ENABLE_SHIFT_PER_TARGET) +
+                                                        (source >> 3));
+  uint8_t current = *current_ptr;
+  current = current | ( 1 << (source & 0x7));
+  *current_ptr = current;
+
+}
+
+void PLIC_disable_interrupt (plic_instance_t * this_plic, plic_source source){
+  
+  unsigned long hart_id = read_csr(mhartid);
+  volatile uint8_t * current_ptr = (volatile uint8_t *) (this_plic->base_addr +
+                                                         PLIC_ENABLE_OFFSET +
+                                                         (hart_id << PLIC_ENABLE_SHIFT_PER_TARGET) +
+                                                         (source >> 3));
+  uint8_t current = *current_ptr;
+  current = current & ~(( 1 << (source & 0x7)));
+  *current_ptr = current;
+  
+}
+
+void PLIC_set_priority (plic_instance_t * this_plic, plic_source source, plic_priority priority){
+
+  if (this_plic->num_priorities > 0) {
+    volatile plic_priority * priority_ptr = (volatile plic_priority *)
+      (this_plic->base_addr +
+       PLIC_PRIORITY_OFFSET +
+       (source << PLIC_PRIORITY_SHIFT_PER_SOURCE));
+    *priority_ptr = priority;
+  }
+}
+
+plic_source PLIC_claim_interrupt(plic_instance_t * this_plic){
+  
+  unsigned long hart_id = read_csr(mhartid);
+
+  volatile plic_source * claim_addr = (volatile plic_source * )
+    (this_plic->base_addr +
+     PLIC_CLAIM_OFFSET +
+     (hart_id << PLIC_CLAIM_SHIFT_PER_TARGET));
+
+  return  *claim_addr;
+  
+}
+
+void PLIC_complete_interrupt(plic_instance_t * this_plic, plic_source source){
+  
+  unsigned long hart_id = read_csr(mhartid);
+  volatile plic_source * claim_addr = (volatile plic_source *) (this_plic->base_addr +
+                                                                PLIC_CLAIM_OFFSET +
+                                                                (hart_id << PLIC_CLAIM_SHIFT_PER_TARGET));
+  *claim_addr = source;
+  
+}
+
diff --git a/software/shared/drivers_sifive/plic.h b/software/shared/drivers_sifive/plic.h
new file mode 100644
index 0000000..14118a5
--- /dev/null
+++ b/software/shared/drivers_sifive/plic.h
@@ -0,0 +1,70 @@
+// See LICENSE for license details.
+
+#ifndef PLIC_H
+#define PLIC_H
+
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <unistd.h>
+
+
+#include "encoding.h"
+
+// 32 bits per source
+#define PLIC_PRIORITY_OFFSET 0x0000UL
+#define PLIC_PRIORITY_SHIFT_PER_SOURCE 2
+// 1 bit per source (1 address)
+#define PLIC_PENDING_OFFSET  0x1000UL
+#define PLIC_PENDING_SHIFT_PER_SOURCE 0
+
+//0x80 per target
+#define PLIC_ENABLE_OFFSET   0x2000UL
+#define PLIC_ENABLE_SHIFT_PER_TARGET 7
+
+
+#define PLIC_THRESHOLD_OFFSET 0x200000UL
+#define PLIC_CLAIM_OFFSET     0x200004UL
+#define PLIC_THRESHOLD_SHIFT_PER_TARGET 12
+#define PLIC_CLAIM_SHIFT_PER_TARGET     12
+
+#define PLIC_MAX_SOURCE 1023
+#define PLIC_SOURCE_MASK 0x3FF
+
+#define PLIC_MAX_TARGET 15871
+#define PLIC_TARGET_MASK 0x3FFF
+
+
+typedef struct __plic_instance_t
+{
+  uintptr_t base_addr;
+
+  uint32_t num_sources;
+  uint32_t num_priorities;
+  
+} plic_instance_t;
+
+typedef uint32_t plic_source;
+typedef uint32_t plic_priority;
+typedef uint32_t plic_threshold;
+
+void PLIC_init (
+                plic_instance_t * this_plic,
+                uintptr_t             base_addr,
+                uint32_t num_sources,
+                uint32_t num_priorities
+                );
+  
+void PLIC_enable_interrupt (plic_instance_t * this_plic, plic_source source);
+
+void PLIC_disable_interrupt (plic_instance_t * this_plic, plic_source source);
+
+void PLIC_set_priority (plic_instance_t * this_plic, plic_source source, plic_priority priority);
+
+plic_source PLIC_claim_interrupt(plic_instance_t * this_plic);
+
+void PLIC_complete_interrupt(plic_instance_t * this_plic, plic_source source);
+
+void PLIC_set_threshold(plic_instance_t * this_plic, plic_threshold threshold);
+
+#endif
diff --git a/software/shared/entry.S b/software/shared/entry.S
new file mode 120000
index 0000000..11f412b
--- /dev/null
+++ b/software/shared/entry.S
@@ -0,0 +1 @@
+../../riscv-tests/debug/programs/entry.S
\ No newline at end of file
diff --git a/software/shared/gpio.h b/software/shared/gpio.h
new file mode 100644
index 0000000..ed48255
--- /dev/null
+++ b/software/shared/gpio.h
@@ -0,0 +1,10 @@
+// See LICENSE for license details.
+
+#ifndef SIFIVE_GPIO_H
+#define SIFIVE_GPIO_H
+
+#define GPIO_IN_OFFSET 0
+#define GPIO_OUT_OFFSET 0
+#define GPIO_TRI_OFFSET 4
+
+#endif
diff --git a/software/shared/init.c b/software/shared/init.c
new file mode 100644
index 0000000..ab790b2
--- /dev/null
+++ b/software/shared/init.c
@@ -0,0 +1,48 @@
+// See LICENSE for license details.
+
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#include "encoding.h"
+#include "shared.h"
+
+int main(void);
+
+#ifdef USE_PLIC
+extern void handle_m_ext_interrupt();
+#endif
+
+
+#ifdef USE_M_TIME
+extern void handle_m_time_interrupt();
+#endif
+
+
+void _init(void)
+{
+  
+  exit(main());
+}
+
+uintptr_t handle_trap(uintptr_t mcause, uintptr_t epc)
+{
+  if (0){
+#ifdef USE_PLIC
+  // External Machine-Level Interrupt from PLIC
+ }else if ((mcause & MCAUSE_INT) && ((mcause & MCAUSE_CAUSE)  == IRQ_M_EXT)) {
+    handle_m_ext_interrupt();
+#endif
+#ifdef USE_M_TIME
+  // External Machine-Level Interrupt from PLIC
+ }else if ((mcause & MCAUSE_INT) && ((mcause & MCAUSE_CAUSE)  == IRQ_M_TIMER)) {
+    handle_m_time_interrupt();
+#endif
+  }    
+  else{
+    write(1, "trap\n", 5);
+    _exit(1 + mcause);
+  }
+  return epc;
+}
diff --git a/software/shared/link.lds b/software/shared/link.lds
new file mode 120000
index 0000000..013b6cb
--- /dev/null
+++ b/software/shared/link.lds
@@ -0,0 +1 @@
+../../riscv-tests/debug/targets/m2gl_m2s/link.lds
\ No newline at end of file
diff --git a/software/shared/shared.h b/software/shared/shared.h
new file mode 100644
index 0000000..e8f3bf4
--- /dev/null
+++ b/software/shared/shared.h
@@ -0,0 +1,73 @@
+// See LICENSE for license details.
+
+#ifndef SIFIVE_SHARED_H
+#define SIFIVE_SHARED_H
+
+#include "uart.h"
+#include "gpio.h"
+#include "plic.h"
+
+// Some things missing from the official encoding.h
+#define MCAUSE_INT         0x80000000
+#define MCAUSE_CAUSE       0x7FFFFFFF
+
+/****************************************************************************
+ * Platform definitions
+ *****************************************************************************/
+
+#define MTIMECMP_BASE_ADDR     0x44004000UL
+#define MTIME_ADDR             0x4400BFF8UL
+#define PLIC_BASE_ADDR         0x40000000UL
+#define UART_BASE_ADDR         0x48000000UL
+#define GPIO_BASE_ADDR         0x48002000UL
+#define SPI_BASE_ADDR          0x48001000UL
+
+
+/****************************************************************************
+ * Clock Parameters
+ *****************************************************************************/
+
+#define RTC_PRESCALER 100
+#define CLOCK_FREQUENCY 62500000
+
+/****************************************************************************
+ * GPIO Connections
+ *****************************************************************************/
+
+
+// Each of these OFFSETS holds 4 bits.
+
+#define RED_LEDS_OFFSET   0
+#define GREEN_LEDS_OFFSET 4
+#define BLUE_LEDS_OFFSET  8
+#define JA_OUT_OFFSET     12 // JA Pins 1-4 are outputs.
+
+#define BUTTONS_OFFSET    16
+#define SWITCHES_OFFSET   20
+#define JA_IN_OFFSET      24 // JA Pins 7-10 are inputs.
+// reserved input offset   38 // Tied to zero.
+
+/****************************************************************************
+ * External Interrupts handled by PLIC.
+ *****************************************************************************/
+
+// Interrupt devices
+#define INT_DEVICE_BUTTON_0 1
+#define INT_DEVICE_BUTTON_1 2
+#define INT_DEVICE_BUTTON_2 3
+#define INT_DEVICE_BUTTON_3 4
+
+#define INT_DEVICE_JA_7 5
+#define INT_DEVICE_JA_8 6
+#define INT_DEVICE_JA_9 7
+#define INT_DEVICE_JA_10 8
+
+
+// Setting these correctly makes the initialization scripts
+// run faster.
+#define PLIC_NUM_SOURCES 31
+#define PLIC_NUM_PRIORITIES 0
+
+void write_hex(int fd, uint32_t hex);
+
+#endif
diff --git a/software/shared/spi.h b/software/shared/spi.h
new file mode 100644
index 0000000..9c974c1
--- /dev/null
+++ b/software/shared/spi.h
@@ -0,0 +1,8 @@
+// See LICENSE for license details.
+
+#ifndef SIFIVE_SPI_H
+#define SIFIVE_SPI_H
+
+//TODO
+
+#endif
diff --git a/software/shared/syscall.c b/software/shared/syscall.c
new file mode 100644
index 0000000..2e4199f
--- /dev/null
+++ b/software/shared/syscall.c
@@ -0,0 +1,206 @@
+// See LICENSE for license details.
+
+/* This is an incomplete version of a syscall library, 
+ * which really only supports simple reads and writes over UART.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <stddef.h>
+#include <unistd.h>
+#include <errno.h>
+#include <sys/stat.h>
+#include <sys/times.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "encoding.h"
+
+#include "shared.h"
+
+volatile uint64_t tohost __attribute__((aligned(64)));
+volatile uint64_t fromhost __attribute__((aligned(64)));
+
+void write_hex(int fd, uint32_t hex){
+  uint8_t ii;
+  uint8_t jj;
+  char towrite;
+  write( fd , "0x", 2 );
+  for (ii = 8 ; ii > 0; ii--){
+    jj = ii-1;
+    uint8_t digit = ((hex & (0xF << (jj*4))) >> (jj*4));
+    towrite = digit < 0xA ? ('0' + digit) : ('A' +  (digit - 0xA));
+    write( fd, &towrite, 1);
+  }
+
+}
+
+               
+void _exit(int code)
+{
+  volatile uint32_t* leds = (uint32_t*) (GPIO_BASE_ADDR + GPIO_OUT_OFFSET);
+  const char * message = "\nProgam has exited with code:";
+  
+  *leds = (~(code));
+
+  write(STDERR_FILENO, message, strlen(message));
+  write_hex(STDERR_FILENO, code);
+
+  while (1);
+    
+}
+
+void *sbrk(ptrdiff_t incr)
+{
+  //  extern char _end[];
+  //  extern char _heap_end[];
+  //  static char *curbrk = _end;
+
+  //  if ((curbrk + incr < _end) || (curbrk + incr > _heap_end))
+    return NULL - 1;
+
+    //  curbrk += incr;
+    //return curbrk - incr;
+}
+
+static int stub(int err)
+{
+  errno = err;
+  return -1;
+}
+
+int open(const char* name, int flags, int mode)
+{
+  return stub(ENOENT);
+}
+
+int openat(int dirfd, const char* name, int flags, int mode)
+{
+  return stub(ENOENT);
+}
+
+int close(int fd)
+{
+  return stub(EBADF);
+}
+
+int execve(const char* name, char* const argv[], char* const env[])
+{
+  return stub(ENOMEM);
+}
+
+int fork()
+{
+  return stub(EAGAIN);
+}
+
+int fstat(int fd, struct stat *st)
+{
+  if (isatty(fd)) {
+    st->st_mode = S_IFCHR;
+    return 0;
+  }
+
+  return stub(EBADF);
+}
+
+int getpid()
+{
+  return 1;
+}
+
+int isatty(int fd)
+{
+  if (fd == STDOUT_FILENO || fd == STDERR_FILENO)
+    return 1;
+
+  errno = EBADF;
+  return 0;
+}
+
+int kill(int pid, int sig)
+{
+  return stub(EINVAL);
+}
+
+int link(const char *old_name, const char *new_name)
+{
+  return stub(EMLINK);
+}
+
+off_t lseek(int fd, off_t ptr, int dir)
+{
+  if (isatty(fd))
+    return 0;
+
+  return stub(EBADF);
+}
+
+ssize_t read(int fd, void* ptr, size_t len)
+{
+  int i;
+  uint8_t * current = (uint8_t*) ptr;
+  volatile uint8_t  * uart_rx = (uint8_t*) (UART_BASE_ADDR + UART_RX_OFFSET);
+  volatile uint32_t * uart_rx_cnt = (uint32_t*) (UART_BASE_ADDR + UART_RX_COUNT_OFFSET);
+
+  ssize_t result = 0;
+  
+  if (isatty(fd)) {
+    
+    for (current = (uint8_t*) ptr;
+	 (current < ((uint8_t*) ptr) + len) && (*uart_rx_cnt > 0);
+	 current ++){
+      *current = *uart_rx;
+      result ++;
+    }
+    return result;
+
+  }
+ 
+  return stub(EBADF);
+}
+
+int stat(const char* file, struct stat* st)
+{
+  return stub(EACCES);
+}
+
+clock_t times(struct tms* buf)
+{
+  return stub(EACCES);
+}
+
+int unlink(const char* name)
+{
+  return stub(ENOENT);
+}
+
+int wait(int* status)
+{
+  return stub(ECHILD);
+}
+
+ssize_t write(int fd, const void* ptr, size_t len)
+{
+
+  const uint8_t * current = (const char*) ptr;
+  volatile uint32_t * uart_tx_count = (uint32_t*) (UART_BASE_ADDR + UART_TX_COUNT_OFFSET);
+  volatile uint8_t *  uart_tx = (uint8_t*) (UART_BASE_ADDR + UART_TX_OFFSET);
+
+  size_t jj;
+  if (isatty(fd)) {
+    
+    for (jj = 0; jj < len; jj++){
+
+      while ((*uart_tx_count) < 1);
+      *uart_tx = current[jj];
+
+      if (current[jj] == '\n'){
+	while ((*uart_tx_count) < 1);
+	*uart_tx = '\r';
+      }
+    }
+    return len;
+  } 
+  return stub(EBADF);
+}
diff --git a/software/shared/uart.h b/software/shared/uart.h
new file mode 100644
index 0000000..e97ef8b
--- /dev/null
+++ b/software/shared/uart.h
@@ -0,0 +1,12 @@
+// See LICENSE for license details.
+
+#ifndef SIFIVE_UART_H
+#define SIFIVE_UART_H
+
+#define UART_RX_OFFSET 0
+#define UART_TX_OFFSET 0
+#define UART_TX_COUNT_OFFSET 0x4
+#define UART_RX_COUNT_OFFSET 0x8
+#define UART_DIVIDER_OFFSET  0xC
+
+#endif