serial: headless mode tying serial port to stdio for OS debugging

- --headless: no rendering/TUI; serial TX -> stdout, RX <- stdin (nonblocking)
- generalize serial: serial_out_fd/serial_in_fd on the GB; internal-clock
  transfer now shifts in a real RX byte (from stdin) instead of always 0xFF
- paced to native speed by default, --uncapped for turbo; pipes/scripts cleanly
- README: document headless serial console

1 files changed, 18 insertions, 3 deletions

diff --git a/src/gb.c b/src/gb.c
index afdcb89..05429fd 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -4,6 +4,7 @@
 #include "cpu.h"
 #include <string.h>
 #include <stdio.h>
+#include <unistd.h>
 
 void gb_request_interrupt(GB *gb, u8 flag) {
     gb->iff |= flag;
@@ -116,9 +117,21 @@ void gb_write(GB *gb, u16 addr, u8 val) {
     if (addr == 0xFF02) {
         gb->sc = val;
         if ((val & 0x81) == 0x81) {
-            // transfer with internal clock: log & complete instantly
-            if (gb->serial_log) { fputc(gb->sb, stderr); fflush(stderr); }
-            gb->sb = 0xFF;
+            // transfer with internal clock (GB is master): shift out gb->sb,
+            // shift in a byte from the peer. Completes instantly.
+            u8 outb = gb->sb;
+            if (gb->serial_log) { fputc(outb, stderr); fflush(stderr); }
+            if (gb->serial_out_fd >= 0) {
+                unsigned char c = outb;
+                (void)!write(gb->serial_out_fd, &c, 1);
+            }
+            // received byte: from serial_in_fd if available, else open-bus 0xFF
+            u8 inb = 0xFF;
+            if (gb->serial_in_fd >= 0) {
+                unsigned char c;
+                if (read(gb->serial_in_fd, &c, 1) == 1) inb = c;
+            }
+            gb->sb = inb;
             gb->sc &= ~0x80;
             gb_request_interrupt(gb, INT_SERIAL);
         }
@@ -204,6 +217,8 @@ void gb_reset(GB *gb) {
         c->af = 0x01B0; c->bc = 0x0013; c->de = 0x00D8; c->hl = 0x014D;
     }
     c->ime = false;
+    gb->serial_out_fd = -1;
+    gb->serial_in_fd = -1;
     gb->wram_bank = 1;
     gb->joyp_sel = 0x30;
     gb->iff = 0xE1;