diff --git a/litex/soc/cores/cpu/openc906/irq.h b/litex/soc/cores/cpu/openc906/irq.h
index 9d9e4af17..8f4580b13 100644
--- a/litex/soc/cores/cpu/openc906/irq.h
+++ b/litex/soc/cores/cpu/openc906/irq.h
@@ -18,6 +18,8 @@ extern "C" {
 #define PLIC_THRSHLD 0x90200000L // Per-pin priority must be >= this to trigger
 #define PLIC_CLAIM   0x90200004L // Claim & completion register address
 
+#define PLIC_EXT_IRQ_BASE 16
+
 static inline unsigned int irq_getie(void)
 {
 	return (csrr(mstatus) & CSR_MSTATUS_MIE) != 0;
diff --git a/litex/soc/cores/cpu/rocket/irq.h b/litex/soc/cores/cpu/rocket/irq.h
index 9548b01fd..83f854c86 100644
--- a/litex/soc/cores/cpu/rocket/irq.h
+++ b/litex/soc/cores/cpu/rocket/irq.h
@@ -18,6 +18,8 @@ extern "C" {
 #define PLIC_THRSHLD 0x0c200000L // Per-pin priority must be >= this to trigger
 #define PLIC_CLAIM   0x0c200004L // Claim & completion register address
 
+#define PLIC_EXT_IRQ_BASE 1
+
 static inline unsigned int irq_getie(void)
 {
 	return (csrr(mstatus) & CSR_MSTATUS_MIE) != 0;
diff --git a/litex/soc/software/libbase/isr.c b/litex/soc/software/libbase/isr.c
index 98679f831..eb2fe7ae5 100644
--- a/litex/soc/software/libbase/isr.c
+++ b/litex/soc/software/libbase/isr.c
@@ -30,35 +30,37 @@ void isr(void)
   }
 }
 #elif defined(__rocket__) || defined(__openc906__)
-#if defined(__openc906__)
-#define PLIC_EXT_IRQ_BASE 16
-#else
-#define PLIC_EXT_IRQ_BASE 1
-#endif
+
+// PLIC initialization.
 void plic_init(void);
 void plic_init(void)
 {
 	int i;
 
-	// priorities for first 8 external interrupts
+	// Set priorities for the first 8 external interrupts to 1.
 	for (i = 0; i < 8; i++)
 		*((unsigned int *)PLIC_BASE + PLIC_EXT_IRQ_BASE + i) = 1;
-	// enable first 8 external interrupts
+
+	// Enable the first 8 external interrupts
 	*((unsigned int *)PLIC_ENABLED) = 0xff << PLIC_EXT_IRQ_BASE;
-	// set priority threshold to 0 (any priority > 0 triggers interrupt)
+
+	// Set priority threshold to 0 (any priority > 0 triggers an interrupt).
 	*((unsigned int *)PLIC_THRSHLD) = 0;
 }
 
+// Interrupt Service Routine.
 void isr(void)
 {
 	unsigned int claim;
 
+	// Claim and handle pending interrupts.
 	while ((claim = *((unsigned int *)PLIC_CLAIM))) {
 		switch (claim - PLIC_EXT_IRQ_BASE) {
 		case UART_INTERRUPT:
-			uart_isr();
+			uart_isr(); // Handle UART interrupt.
 			break;
 		default:
+			// Unhandled interrupt source, print diagnostic information.
 			printf("## PLIC: Unhandled claim: %d\n", claim);
 			printf("# plic_enabled:    %08x\n", irq_getmask());
 			printf("# plic_pending:    %08x\n", irq_pending());
@@ -70,6 +72,7 @@ void isr(void)
 			printf("###########################\n\n");
 			break;
 		}
+		// Acknowledge the interrupt.
 		*((unsigned int *)PLIC_CLAIM) = claim;
 	}
 }