An update.

Makefile

@@ -5,8 +5,10 @@ CFLAGS = -flto -ffreestanding -fno-builtin -fno-pie -Wall -std=c2x -m16 -nostdli
 %.o: %.c
 	$(CC) $^ -o $@ $(CFLAGS)
 
-all: $(SRCS:.c=.o)
+bin: $(SRCS:.c=.o)
 	$(CC) $(LDFLAGS) -o bin $^
 
+all: bin
+
 clean:
 	$(RM) $(SRCS:.c=.o) bin

boot.c

@@ -1,4 +1,6 @@
+#include <stdlib.h>
 #include <stdbool.h>
+#include "hedley.h"
 
 #define BOARD ((Cell*)0x7F00)
 
@@ -8,55 +10,34 @@ typedef enum : char {
 	NOT   = 'O'
 } Cell;
 
-inline void putchar(char);
-inline char readchar(void);
-inline void printboard(void);
-inline bool checkwin(Cell c);
+HEDLEY_ALWAYS_INLINE char readchar(void);
+HEDLEY_ALWAYS_INLINE bool checkwin(Cell c);
+HEDLEY_NEVER_INLINE void putchar(char);
+HEDLEY_NEVER_INLINE void printboard(void);
+
+char numpadkeys[];
 
 void _start(void) {
 	Cell cur = CROSS;
 	for (char i = 0; i < 3; i++)
 		for (char j = 0; j < 3; j++)
-			*(BOARD + i * 3 + j) = NONE;
+			BOARD[i * 3 + j] = NONE;
 	while (1) {
 		printboard();
-		char c;
-		switch (readchar()) {
-			case 0x47: // 7
-				c = 0;
-				break;
-			case 0x48: // 8
-				c = 1;
-				break;
-			case 0x49: // 9
-				c = 2;
-				break;
-
-			case 0x4B: // 4
-				c = 3;
-				break;
-			case 0x4C: // 5
-				c = 4;
-				break;
-			case 0x4D: // 6
-				c = 5;
-				break;
-
-			case 0x4F: // 1
-				c = 6;
-				break;
-			case 0x50: // 2
-				c = 7;
-				break;
-			case 0x51: // 3
-				c = 8;
-				break;
-			default:
+		char k = readchar();
+		char o;
+		if      (k > 0x46)
+			o = 0x47;
+		else if (k < 0x13)
+			o =  0x10;
+		else if (k < 0x21)
+			o = (0x1E - 4);
+		else
+			o = (0x2C - 8);
+		size_t c = numpadkeys[k - o];
+		if (BOARD[c] != NONE)
 			continue;
-		}
-		if (*(BOARD + c) != NONE)
-			continue;
-		*(BOARD + c) = cur;
+		BOARD[c] = cur;
 		if (checkwin(cur))
 			break;
 		if (cur == CROSS)
@@ -68,9 +49,14 @@ void _start(void) {
 	printboard();
 	while(1)
 		;
+	HEDLEY_UNREACHABLE();
 }
+// numpadkeys[x - 0x47]
+char numpadkeys[] = {0, 1, 2, false, 3, 4, 5, false, 6, 7, 8};
 
-inline void printboard(void) {
+
+HEDLEY_NEVER_INLINE
+void printboard(void) {
 	__asm__ volatile (
 		"movb $0x02, %%ah;"
 		"movb $0x00, %%bh;"
@@ -82,28 +68,35 @@ inline void printboard(void) {
 	);
 	for (char i = 0; i < 3; i++) {
 		for (char j = 0; j < 3; j++) {
-			putchar(*(BOARD + i * 3 + j));
+			putchar(BOARD[i * 3 + j]);
 		}
 		putchar('\n');
 		putchar('\r');
 	}
 }
 
-inline bool checkwin(Cell c) {
-	for (char i = 0; i < 3; i++)
-		if ((*(BOARD + i) == c) && (*(BOARD + i + 3) == c) && (*(BOARD + i + 6) == c))
+HEDLEY_ALWAYS_INLINE
+bool checkwin(Cell c) {
+	for (size_t i = 0; i <= 2; i++)
+		if ((BOARD[i] == c) && (BOARD[i + 3] == c) && (BOARD[i + 6] == c))
 			return true;
-	for (char i = 0; i < 3; i++)
-		if ((*(BOARD + 3*i) == c) && (*(BOARD + 3*i + 1) == c) && (*(BOARD + 3*i + 2) == c))
+	for (size_t i = 0; i <= 2; i++)
+		if ((BOARD[3*i] == c) && (BOARD[3*i + 1] == c) && (BOARD[3*i + 2] == c))
 			return true;
-	if (*(BOARD) == c && *(BOARD + 4) == c && *(BOARD + 8))
+	if (BOARD[0] == c && BOARD[4] == c && BOARD[8] == c)
 		return true;
-	if (*(BOARD + 2) == c && *(BOARD + 4) == c && *(BOARD + 7))
+	if (BOARD[2] == c && BOARD[4] == c && BOARD[6] == c)
 		return true;
 	return false;
 }
+/*
+ * 012
+ * 345
+ * 678
+ */
 
-inline char readchar(void) {
+HEDLEY_ALWAYS_INLINE
+char readchar(void) {
 	char out;
 __asm__ volatile (
 	"movb $0x00, %%ah;"
@@ -116,7 +109,8 @@ __asm__ volatile (
 	return out;
 }
 
-inline void putchar(char out) {
+HEDLEY_NEVER_INLINE
+void putchar(char out) {
 __asm__ volatile (
 	"movb $0x0e, %%ah;"
 	"movb %0, %%al;"

linker.ld

@@ -23,5 +23,6 @@ SECTIONS
     /DISCARD/ : { *(.eh_frame)
               *(.comment)
               *(.note.GNU-stack)
+	      *(.note.gnu.build-id)
             }
 }