diff --git a/include/panic.h b/include/panic.h
index 4f8592e..dc96da1 100644
--- a/include/panic.h
+++ b/include/panic.h
@@ -4,6 +4,7 @@
 #define _PANIC_STR2(x) #x
 
 #define panic(msg) _panic_impl(_PANIC_STR(__LINE__), __FILE__, msg)
+#define kassert(val, msg) do { if (!(val)) { panic(msg); } } while(0)
 
 _Noreturn void _panic_impl(char *line, char *file, char *msg);
 
diff --git a/src/arch/amd64/debugger.c b/src/arch/amd64/debugger.c
new file mode 100644
index 0000000..4df59f7
--- /dev/null
+++ b/src/arch/amd64/debugger.c
@@ -0,0 +1,343 @@
+#include <lib.h>
+#include <stddef.h>
+#include <serial.h>
+#include <backtrace.h>
+#include "debugger.h"
+
+struct dr6 {
+	uint64_t b0 : 1,
+	         b1 : 1,
+			 b2 : 1,
+			 b3 : 1,
+			 : 7,
+			 bld : 1,
+			 : 1,
+			 bd : 1,
+			 bs : 1,
+			 bt : 1,
+			 rtm : 1,
+			 : 47;
+};
+
+struct dr7 {
+	uint64_t l0 : 1,
+	         g0 : 1,
+			 l1 : 1,
+			 g1 : 1,
+			 l2 : 1,
+			 g2 : 1,
+			 l3 : 1,
+			 g3 : 1,
+			 : 8,
+			 rw0 : 2,
+			 len0 : 2,
+			 rw1 : 2,
+			 len1 : 2,
+			 rw2 : 2,
+			 len2 : 2,
+			 rw3 : 2,
+			 len3 : 2,
+			 : 32;
+};
+
+struct rflags {
+	uint64_t cf : 1,
+			 : 1,
+			 pf : 1,
+			 : 1,
+			 af : 1,
+			 : 1,
+			 zf : 1,
+			 sf : 1,
+
+			 tf : 1,
+			 if_ : 1,
+             df : 1,
+			 of : 1,
+			 iopl : 2,
+             nt : 1,
+			 md : 1,
+
+			 rf : 1,
+			 vm : 1,
+			 ac : 1,
+			 vif : 1,
+			 vip : 1,
+			 id : 1,
+			 : 42;
+};
+
+struct breakpoint {
+	uint64_t addr;
+	uint8_t len;
+	uint8_t rw;
+	uint8_t used;
+	uint8_t enable;
+};
+
+static size_t dbg_steps = 0;
+static int dbg_continue = 0;
+static struct breakpoint bkps[4] = {{0}, {0}, {0}, {0}};
+
+static void dbg_putc(char c) {
+	serial_out(c);
+}
+
+static void dbg_puts(char *msg) {
+	serial_out_str(msg);
+}
+
+static char dbg_getc() {
+	return serial_in();
+}
+
+static void debugger_msg(int cause, struct dr6 dr6) {
+	char buf[24];
+	switch (cause) {
+	case DEBUG_INT3:
+		dbg_puts("dbg: reached int3");
+		break;
+	case DEBUG_FAULT:
+		dbg_puts("dbg: a fault occured");
+		break;
+	case DEBUG_DBG:
+		if (dr6.bs)
+			dbg_puts("dbg: finished steps");
+		else if (dr6.b0)
+			dbg_puts("dbg: reached breakpoint 0");
+		else if (dr6.b1)
+			dbg_puts("dbg: reached breakpoint 1");
+		else if (dr6.b2)
+			dbg_puts("dbg: reached breakpoint 2");
+		else if (dr6.b3)
+			dbg_puts("dbg: reached breakpoint 3");
+	}
+	if (dbg_steps > 0) {
+		ultoa(dbg_steps, buf, 10);
+		dbg_puts(" (");
+		dbg_puts(buf);
+		dbg_puts(" steps left)");
+	}
+	if (dbg_continue > 0) {
+		dbg_puts(" (paused continue)");
+	}
+	dbg_puts("\n");
+}
+
+static void debugger_print_reg(char *name, uint64_t val) {
+	char buf[24];
+	char *end;
+	dbg_puts(name);
+	dbg_puts(": 0x");
+
+	end = ultoa(val, buf, 16);
+	for (int i = 0; i < 16 - (end - buf); i++) {
+		dbg_putc('0');
+	}
+	dbg_puts(buf);
+
+	dbg_puts(" (");
+	end = ultoa(val, buf, 10);
+	dbg_puts(buf);
+	dbg_puts(")\n");
+}
+
+static void debugger_print_regs(struct isr_regs *state) {
+	debugger_print_reg("rax", state->rax);
+	debugger_print_reg("rbx", state->rbx);
+	debugger_print_reg("rcx", state->rcx);
+	debugger_print_reg("rdx", state->rdx);
+	debugger_print_reg("rsi", state->rsi);
+	debugger_print_reg("rdi", state->rdi);
+	debugger_print_reg("rsp", state->rsp);
+	debugger_print_reg("rbp", state->rbp);
+	debugger_print_reg("r8 ", state->r8 );
+	debugger_print_reg("r9 ", state->r9 );
+	debugger_print_reg("r10", state->r10);
+	debugger_print_reg("r11", state->r11);
+	debugger_print_reg("r12", state->r12);
+	debugger_print_reg("r13", state->r13);
+	debugger_print_reg("r14", state->r14);
+	debugger_print_reg("r15", state->r15);
+	dbg_puts("---\n");
+	debugger_print_reg("rip", state->rip);
+	dbg_puts("---\n");
+	debugger_print_reg("rflags", state->rflags);
+	struct rflags *rflags = (struct rflags *)state->rflags;
+	dbg_puts("rflags: ");
+	if(rflags->cf)   dbg_puts("CF ");
+	if(rflags->pf)   dbg_puts("PF ");
+	if(rflags->af)   dbg_puts("AF ");
+	if(rflags->zf)   dbg_puts("ZF ");
+	if(rflags->sf)   dbg_puts("SF ");
+	if(rflags->tf)   dbg_puts("TF ");
+	if(rflags->if_)  dbg_puts("IF ");
+	if(rflags->df)   dbg_puts("DF ");
+	if(rflags->of)   dbg_puts("OF ");
+	if(rflags->iopl) dbg_puts("IOPL ");
+	if(rflags->nt)   dbg_puts("NT ");
+	if(rflags->md)   dbg_puts("MD ");
+	if(rflags->rf)   dbg_puts("RF ");
+	if(rflags->vm)   dbg_puts("VM ");
+	if(rflags->ac)   dbg_puts("AC ");
+	if(rflags->vif)  dbg_puts("VIF ");
+	if(rflags->vip)  dbg_puts("VIP ");
+	if(rflags->id)   dbg_puts("ID ");
+	dbg_putc('\n');
+}
+
+#define PROMPT_LEN 60
+
+static int debugger_handle_bkp_cmd(char *buf) {
+	if (buf[0] == 'l') {
+		// list breakpoints
+		char buf[20];
+		for (int i = 0; i < 4; i++) {
+			dbg_puts("breakpoint ");
+			dbg_putc('0' + i);
+			dbg_puts(": ");
+			struct breakpoint bkp = bkps[i];
+			if(!bkp.used) {
+				dbg_puts("unset\n");
+				continue;
+			}
+			dbg_puts(" 0x");
+			ultoa(bkp.addr, buf, 16);
+			dbg_puts(buf);
+			dbg_puts(" len=");
+			switch (bkp.len) {
+				case 0x0: dbg_putc('1'); break;
+				case 0x1: dbg_putc('2'); break;
+				case 0x2: dbg_putc('8'); break;
+				case 0x3: dbg_putc('4'); break;
+			}
+			dbg_puts(" trigger=");
+			switch (bkp.rw) {
+				case 0x0: dbg_puts("x "); break;
+				case 0x1: dbg_puts("w "); break;
+				case 0x2: dbg_puts("io"); break;
+				case 0x3: dbg_puts("rw"); break;
+			}
+			dbg_puts(bkp.enable ? " enabled" : " disabled");
+		}
+		return 1;
+	} else if (buf[0] == 'd') {
+		// disable breakpoints
+		bkps[0].enable = 0;
+		bkps[1].enable = 0;
+		bkps[2].enable = 0;
+		bkps[3].enable = 0;
+		return 1;
+	} else if (buf[0] == 'c') {
+		// clear breakpoints
+		bkps[0].used = 0;
+		bkps[1].used = 0;
+		bkps[2].used = 0;
+		bkps[3].used = 0;
+		return 1;
+	} else if (buf[0] <= '0' && buf[0] >= '3') {
+		dbg_puts("invalid breakpoint command\n");
+		return 1;
+	}
+	// TODO set breakpoints
+	return 1;
+}
+
+static void debugger_load_bkps() {
+	struct dr7 dr7;
+	__asm__ volatile ("mov %%dr7, %0" : "=r"(dr7));
+	dr7.g0 = bkps[0].enable & bkps[0].used;
+	dr7.g1 = bkps[1].enable & bkps[0].used;
+	dr7.g2 = bkps[2].enable & bkps[0].used;
+	dr7.g3 = bkps[3].enable & bkps[0].used;
+	dr7.l0 = 0;
+	dr7.l1 = 0;
+	dr7.l2 = 0;
+	dr7.l3 = 0;
+	dr7.rw0 = bkps[0].rw;
+	dr7.rw1 = bkps[1].rw;
+	dr7.rw2 = bkps[2].rw;
+	dr7.rw3 = bkps[3].rw;
+	dr7.len0 = bkps[0].len;
+	dr7.len1 = bkps[1].len;
+	dr7.len2 = bkps[2].len;
+	dr7.len3 = bkps[3].len;
+	__asm__ volatile ("mov %0, %%dr7" : : "r"(dr7));
+}
+
+static int debugger_prompt(struct isr_regs *state) {
+	char buf[PROMPT_LEN];
+	char *p = buf;
+	dbg_puts("dbg> ");
+	while ((*p = dbg_getc()) != '\r' && p < buf + PROMPT_LEN - 1) {
+		dbg_putc(*p++);
+	}
+	dbg_putc('\n');
+	for(; p < buf + PROMPT_LEN; p++) {
+		*p = 0;
+	}
+
+	struct rflags *rflags = (struct rflags *)&state->rflags;
+
+	switch (buf[0]) {
+	case '\0': // nothing entered
+		return 1;
+	case 'h': // help
+		dbg_puts("see debugger.c for help information\n");
+		return 1;
+	case 'q': // quit
+		return 0;
+	case 'c': // continue
+		rflags->tf = 1;
+		dbg_continue = 1;
+		return 0;
+	case 'r': // print registers
+		debugger_print_regs(state);
+		return 1;
+	case 'k': // backtrace
+		log_backtrace_ex((void *)state->rip, (void *)state->rbp);
+		return 1;
+	case 's': // step (n)
+		rflags->tf = 1;
+		dbg_steps = atol(buf + 1) - 1;
+		if (dbg_steps < 0) dbg_steps = 0;
+		return 0;
+	case 'b': // breakpoints
+		{
+			int res = debugger_handle_bkp_cmd(buf+1);
+			debugger_load_bkps();
+			return res;
+		}
+	default:
+		dbg_puts("unknown command\n");
+		return 1;
+	}
+}
+
+void debugger(struct isr_regs *state, int cause) {
+	struct dr6 dr6;
+	__asm__ volatile ("mov %%dr6, %0" : "=r"(dr6));
+
+	if (dr6.b0 || dr6.b1 || dr6.b2 || dr6.b3) {
+
+	} else {
+		if (dr6.bs && dbg_steps > 0) {
+			dbg_steps--;
+			return;
+		}
+
+		if (dr6.bs && dbg_continue) {
+			return;
+		}
+	}
+
+	debugger_msg(cause, dr6);
+
+	dbg_steps = 0;
+	dbg_continue = 0;
+
+	struct rflags *rflags = (struct rflags *)&state->rflags;
+	rflags->tf = 0;
+
+	while (debugger_prompt(state));
+}
diff --git a/src/arch/amd64/debugger.h b/src/arch/amd64/debugger.h
new file mode 100644
index 0000000..e679c07
--- /dev/null
+++ b/src/arch/amd64/debugger.h
@@ -0,0 +1,9 @@
+#pragma once
+
+#include "idt.h"
+
+#define DEBUG_INT3  0x00
+#define DEBUG_DBG   0x01 
+#define DEBUG_FAULT 0x02
+
+void debugger(struct isr_regs *state, int cause);
diff --git a/src/arch/amd64/idt.S b/src/arch/amd64/idt.S
index 33d488a..cd8783e 100644
--- a/src/arch/amd64/idt.S
+++ b/src/arch/amd64/idt.S
@@ -11,9 +11,9 @@ extern idt_pic_eoi
 	push rbx
 	push rcx
 	push rdx
-	push rbp
-	push rdi
 	push rsi
+	push rdi
+	push rbp
 	push r8
 	push r9
 	push r10
@@ -33,9 +33,9 @@ extern idt_pic_eoi
 	pop r10
 	pop r9
 	pop r8
-	pop rsi
-	pop rdi
 	pop rbp
+	pop rdi
+	pop rsi
 	pop rdx
 	pop rcx
 	pop rbx
@@ -51,8 +51,7 @@ isr_stub_%+%1:
 	cld
 	mov rdi, %1                  ; exception number
 	mov rsi, 0                   ; placeholder error code
-	mov rdx, [rsp + 15 * 8]      ; instruction pointer
-	mov rcx, rbp                 ; base pointer for stack trace
+	mov rdx, rsp                 ; top of stack
 	call idt_exception_handler
 	POPALL
 	iretq
@@ -64,15 +63,18 @@ isr_stub_%+%1:
 %macro ISRExceptionCode 1
 align 8
 isr_stub_%+%1:
+	; retrieve the error code without corrupting registers
+	mov [isr_tmp], rax
+	pop rax
+	mov [isr_err_code], rax
+	mov rax, [isr_tmp]
 	PUSHALL
 	cld
 	mov rdi, %1                  ; exception number
-	mov rsi, [rsp + 15 * 8]      ; error code
-	mov rdx, [rsp + 16 * 8]      ; instruction pointer
-	mov rcx, rbp                 ; base pointer for stack trace
+	mov rsi, [isr_err_code]      ; error code
+	mov rdx, rsp                 ; top of stack
 	call idt_exception_handler
 	POPALL
-	sub rsp, 8                   ; discard error code
 	iretq
 %endmacro
 
@@ -188,6 +190,10 @@ PICGeneric  47 ; 15
 	%assign i i+1
 %endrep
 
+section .data
+isr_tmp: dq 0
+isr_err_code: dq 0
+
 ; isr stub table
 section .rodata
 bits 64
diff --git a/src/arch/amd64/idt.c b/src/arch/amd64/idt.c
index 583b8a9..c0ea5a8 100644
--- a/src/arch/amd64/idt.c
+++ b/src/arch/amd64/idt.c
@@ -1,10 +1,10 @@
 #include <stdint.h>
 #include <stddef.h>
-#include <panic.h>
 #include <lib.h>
 #include <serial.h>
+#include <panic.h>
 
-#include "backtrace.h"
+#include "debugger.h"
 #include "idt.h"
 #include "pic.h"
 
@@ -108,7 +108,16 @@ char *EXCEPTIONS[] = {
 	"0x1F Reserved",
 };
 
-void idt_exception_handler(uint64_t exception, uint64_t code, void *rip, void *rbp) {
+void idt_exception_handler(uint64_t exception, uint64_t code, struct isr_regs *state) {
+	// breakpoint interrupt
+	if (exception == 0x03) {
+		debugger(state, DEBUG_INT3);
+		return;
+	} else if (exception == 0x01) {
+		debugger(state, DEBUG_DBG);
+		return;
+	}
+
 	// TODO don't just panic
 	char buf[24];
 	char msg[256] = "Exception ";
@@ -128,7 +137,7 @@ void idt_exception_handler(uint64_t exception, uint64_t code, void *rip, void *r
 		strcat(msg, buf);
 	}
 
-	panic_interrupt(rip, rbp, msg);
+	panic_interrupt((void *)state->rip, (void *)state->rbp, msg);
 }
 
 void idt_pic_eoi(uint8_t exception) {
diff --git a/src/arch/amd64/idt.h b/src/arch/amd64/idt.h
index 5627657..89a9dab 100644
--- a/src/arch/amd64/idt.h
+++ b/src/arch/amd64/idt.h
@@ -1,3 +1,29 @@
 #pragma once
 
+#include <stdint.h>
+
+struct isr_regs {
+	uint64_t r15;
+	uint64_t r14;
+	uint64_t r13;
+	uint64_t r12;
+	uint64_t r11;
+	uint64_t r10;
+	uint64_t r9;
+	uint64_t r8;
+	uint64_t rbp;
+	uint64_t rdi;
+	uint64_t rsi;
+	uint64_t rdx;
+	uint64_t rcx;
+	uint64_t rbx;
+	uint64_t rax;
+
+	uint64_t rip;
+	uint64_t cs;
+	uint64_t rflags;
+	uint64_t rsp;
+	uint64_t ss;
+};
+
 void idt_init();
diff --git a/src/kmain.c b/src/kmain.c
index bb0584f..07244bf 100644
--- a/src/kmain.c
+++ b/src/kmain.c
@@ -4,6 +4,7 @@
 #include <serial.h>
 #include <fb.h>
 #include <shim.h>
+#include <panic.h>
 
 void kmain(struct boot_info *info) {
 	memory_init(&info->map);
diff --git a/src/lib.c b/src/lib.c
index 6c17161..ee4afcb 100644
--- a/src/lib.c
+++ b/src/lib.c
@@ -212,7 +212,7 @@ end:
 		if (n == 0) { \
 			buffer[0] = '0'; \
 			buffer[1] = '\0'; \
-			return buffer; \
+			return buffer + 1; \
 		} \
 		char *start = buffer; \
 		for (; n; n /= radix) { \
@@ -237,7 +237,7 @@ UXTOA(long long int, ulltoa)
 		if (n == 0) { \
 			buffer[0] = '0'; \
 			buffer[1] = '\0'; \
-			return buffer; \
+			return buffer + 1; \
 		} \
 		if (n < 0) { \
 			*buffer++ = '-'; \