From 25727a362ff54dcb05472fbafcdf8ac04702c83b Mon Sep 17 00:00:00 2001
From: cnlohr <lohr85@gmail.com>
Date: Tue, 14 Mar 2023 16:40:30 -0400
Subject: [PATCH] Improve demonstration of using self-modifying code.
---
examples/self_modify_code/self_modify_code.c | 27 +++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)
diff --git a/examples/self_modify_code/self_modify_code.c b/examples/self_modify_code/self_modify_code.c
index 5e40b5f..bf33dc5 100644
--- a/examples/self_modify_code/self_modify_code.c
+++ b/examples/self_modify_code/self_modify_code.c
@@ -8,6 +8,8 @@
uint32_t count;
+// This is a complicated way to do it from C land, as a demonstration
+
// Tell the compiler to put this code in the .data section. That
// will cause the startup code to copy it from flash into RAM where
// it can be easily modified at runtime.
@@ -38,10 +40,14 @@ uint32_t ReadCSRSelfModify( uint16_t whichcsr )
// can know what opcode we want to use, then we can let C tell us
// what register it would like the value in.
//
+ // The fence is needed to make sure the CPU knows to not use
+ // cached instructions.
+ //
// The constraints are "ret" is a "write" register, and register a3
// is going to be clobbered by the assembly code.
asm volatile(
".global readCSRLabel \n"
+ " fence \n"
"readCSRLabel: \n"
" csrrs a3, 0x000, x0 \n"
" addi %[ret], a3, 0 \n"
@@ -51,6 +57,25 @@ uint32_t ReadCSRSelfModify( uint16_t whichcsr )
}
+uint32_t ReadCSRSelfModifySimple( uint16_t whichcsr ) __attribute__(( section(".data"))) __attribute__((noinline));
+uint32_t ReadCSRSelfModifySimple( uint16_t whichcsr )
+{
+ uint32_t ret;
+ uint32_t csrcmd = 0x000026f3 | ( whichcsr << 20);
+ asm volatile(
+ ".global readCSRLabel \n"
+ " la a3, readCSRLabel \n"
+ " sw %[csrcmd], 0(a3) \n"
+ " fence \n"
+ "readCSRLabel: \n"
+ " csrrs a3, 0x000, x0 \n"
+ " addi %[ret], a3, 0 \n"
+ : [ret]"=r"(ret) : [csrcmd]"r"(csrcmd) : "a3" );
+
+ return ret;
+}
+
+
int main()
{
SystemInit48HSI();
@@ -64,7 +89,7 @@ int main()
int i;
for( i = 0x000; i < 0x1000; i++ )
{
- uint32_t rv = ReadCSRSelfModify( i );
+ uint32_t rv = ReadCSRSelfModifySimple( i );
if( rv )
printf( "%03x = %08x\n", i, rv );
}
--
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