From e4e2e65293250496cbcb08ade0c3d9ae23b4c427 Mon Sep 17 00:00:00 2001
From: Graham Edgecombe <gpe@grahamedgecombe.com>
Date: Fri, 15 Dec 2017 20:08:55 +0000
Subject: [PATCH] Port example program to C

---
 .gitignore  |  1 +
 Makefile    | 11 +++++++++--
 progmem.c   | 20 ++++++++++++++++++++
 progmem.lds | 21 +++++++++++++++++++++
 progmem.s   | 18 ------------------
 start.s     | 23 +++++++++++++++++++++++
 6 files changed, 74 insertions(+), 20 deletions(-)
 create mode 100644 progmem.c
 create mode 100644 progmem.lds
 delete mode 100644 progmem.s
 create mode 100644 start.s

diff --git a/.gitignore b/.gitignore
index a4085aa..ead9110 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,5 +8,6 @@
 *.rpt
 *.stat
 /pll.sv
+/progmem
 !.git*
 !.mailmap
diff --git a/Makefile b/Makefile
index a9fd34c..87845da 100644
--- a/Makefile
+++ b/Makefile
@@ -20,6 +20,10 @@ FREQ_PLL = 36
 TARGET   = riscv64-unknown-elf
 AS       = $(TARGET)-as
 ASFLAGS  = -march=rv32i -mabi=ilp32
+LD       = $(TARGET)-ld
+LDFLAGS  = -Tprogmem.lds -melf32lriscv
+CC       = $(TARGET)-gcc
+CFLAGS   = -march=rv32i -mabi=ilp32 -Wall -Wextra -pedantic
 OBJCOPY  = $(TARGET)-objcopy
 
 .PHONY: all clean syntax time stat flash
@@ -27,13 +31,16 @@ OBJCOPY  = $(TARGET)-objcopy
 all: $(BIN)
 
 clean:
-	$(RM) $(BLIF) $(ASC_SYN) $(ASC) $(BIN) $(PLL) progmem_syn.hex progmem.hex progmem.o
+	$(RM) $(BLIF) $(ASC_SYN) $(ASC) $(BIN) $(PLL) progmem_syn.hex progmem.hex progmem.o start.o progmem
 
-progmem.hex: progmem.o
+progmem.hex: progmem
 	$(OBJCOPY) -O srec $< /dev/stdout \
 		| srec_cat - -byte-swap 4 -output - -binary \
 		| xxd -p -c 4 > $@
 
+progmem: progmem.o start.o progmem.lds
+	$(LD) $(LDFLAGS) -o $@ progmem.o start.o
+
 progmem_syn.hex:
 	icebram -g 32 256 > $@
 
diff --git a/progmem.c b/progmem.c
new file mode 100644
index 0000000..d280bab
--- /dev/null
+++ b/progmem.c
@@ -0,0 +1,20 @@
+#include <stdint.h>
+
+#define LEDS        *((volatile uint32_t *) 0x00010000)
+#define UART_BAUD   *((volatile uint32_t *) 0x00020000)
+#define UART_STATUS *((volatile uint32_t *) 0x00020004)
+#define UART_DATA   *((volatile  int32_t *) 0x00020008)
+
+int main() {
+    UART_BAUD = 36000000 / 9600;
+
+    for (;;) {
+        int32_t c;
+        do {
+            c = UART_DATA;
+        } while (c < 0);
+
+        UART_DATA = c;
+        LEDS = c;
+    }
+}
diff --git a/progmem.lds b/progmem.lds
new file mode 100644
index 0000000..89dcf29
--- /dev/null
+++ b/progmem.lds
@@ -0,0 +1,21 @@
+ENTRY(start)
+
+MEMORY {
+    bram (rwx) : ORIGIN = 0x00000000, LENGTH = 0x00002000
+}
+
+SECTIONS {
+    .bram : {
+        start.o(.text);
+        *(.text);
+        *(.data);
+
+        . = ALIGN(4);
+        bss_start = .;
+
+        *(.bss);
+
+        . = ALIGN(4);
+        bss_end = .;
+    } > bram
+}
diff --git a/progmem.s b/progmem.s
deleted file mode 100644
index fce1015..0000000
--- a/progmem.s
+++ /dev/null
@@ -1,18 +0,0 @@
-# set baud rate to 9600
-    li t0, 0x00020000
-    li t1, 3750
-    sw t1, 0(t0)
-
-# read char from the UART
-    li t0, 0x00020008
-    li t1, 0x00010000
-loop:
-    lw t2, 0(t0)
-    bltz t2, loop
-
-# write to the LEDs
-    sw t2, 0(t0)
-
-# echo back to the UART
-    sw t2, 0(t1)
-    j loop
diff --git a/start.s b/start.s
new file mode 100644
index 0000000..3195518
--- /dev/null
+++ b/start.s
@@ -0,0 +1,23 @@
+.extern bss_start
+.extern bss_end
+
+.global start
+start:
+    la t0, bss_start
+    la t1, bss_end
+
+    beq t0, t1, clear_bss_done
+clear_bss:
+    sw x0, 0(t0)
+    addi t0, t0, 4
+    bne t0, t1, clear_bss
+clear_bss_done:
+
+    la sp, stack_top
+    call main
+    j .
+
+.section bss
+.local stack_bottom
+.comm stack_bottom, 256, 16
+stack_top: