serial: clean RX path for headless console

- external-clock transfers (SC=$80) deliver a stdin byte into SB without
  echoing to stdout, and stay pending when no input is available -> programs
  can poll for input cleanly. internal-clock (SC=$81) stays pure TX.

1 files changed, 14 insertions, 8 deletions

diff --git a/src/gb.c b/src/gb.c
index 05429fd..7e6771b 100644
--- a/src/gb.c
+++ b/src/gb.c
@@ -117,23 +117,29 @@ void gb_write(GB *gb, u16 addr, u8 val) {
     if (addr == 0xFF02) {
         gb->sc = val;
         if ((val & 0x81) == 0x81) {
-            // transfer with internal clock (GB is master): shift out gb->sb,
-            // shift in a byte from the peer. Completes instantly.
+            // internal clock (GB is master): transmit gb->sb. Completes now.
             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;
+            gb->sb = 0xFF;              // nothing shifted in on a pure TX
+            gb->sc &= ~0x80;
+            gb_request_interrupt(gb, INT_SERIAL);
+        } else if ((val & 0x81) == 0x80) {
+            // external clock (GB waits for the peer): used as a clean RX path.
+            // Complete only when an input byte is available - and do NOT echo
+            // it to stdout - so a program can poll for input without emitting
+            // junk. Stays pending (bit7 set) while stdin has nothing.
             if (gb->serial_in_fd >= 0) {
                 unsigned char c;
-                if (read(gb->serial_in_fd, &c, 1) == 1) inb = c;
+                if (read(gb->serial_in_fd, &c, 1) == 1) {
+                    gb->sb = c;
+                    gb->sc &= ~0x80;
+                    gb_request_interrupt(gb, INT_SERIAL);
+                }
             }
-            gb->sb = inb;
-            gb->sc &= ~0x80;
-            gb_request_interrupt(gb, INT_SERIAL);
         }
         return;
     }