net: SLIP framing over the link port + a host gateway (echo round-trip)

First step of link-port networking. The LCD terminal + OSK freed the serial
port from console duty, so it can be the network link.

Kernel (src/net.asm): raw link-port serial that bypasses the console/
terminal - sys_ssend (transmit, GB drives the clock) and sys_srecv (receive,
GB slave, blocks by yielding). Syscalls 29/30.

Userland: libc ssend()/srecv(); c/necho.c does SLIP (RFC 1055) framing over
them - send a packet, receive the reply, print it.

Host: tools/gateway.py wraps the emulator, owns its link serial, speaks SLIP,
and (for now) echoes every frame back - the "link cable adapter". Console
(ASCII) bytes on the same channel are printed for visibility.

Verified: `necho` sends a SLIP frame, the gateway decodes+echoes it, and gbos
prints the reply - a real framed round-trip over the Game Boy link port.
Next: swap the echo for actual network ops (DNS/HTTP or IRC/chat).

1 files changed, 41 insertions, 0 deletions

diff --git a/src/net.asm b/src/net.asm
new file mode 100644
index 0000000..939b975
--- /dev/null
+++ b/src/net.asm
@@ -0,0 +1,41 @@
+; =============================================================================
+; net.asm - raw link-port serial for networking (bypasses the console/terminal).
+;
+; The Game Boy link port is one 8-bit shift register. These two calls move a
+; byte at a time: ssend transmits with the GB as clock master; srecv waits for
+; a byte with the GB as slave (external clock), yielding while it blocks. On top
+; of these, userland runs SLIP framing and talks to a host-side gateway.
+;
+; The LCD terminal + on-screen keyboard mean the console no longer needs the
+; link port, so it is free to be the network.
+; =============================================================================
+INCLUDE "include/gbos.inc"
+
+SECTION "net", ROM0
+
+; sys_ssend(E = byte) - transmit one byte (GB drives the clock).
+sys_ssend::
+    ld a, e
+    ld [rSB], a
+    ld a, $81                   ; start transfer, internal clock
+    ld [rSC], a
+.wait
+    ld a, [rSC]
+    bit 7, a
+    jr nz, .wait                ; bit7 clears when the byte has shifted out
+    ret
+
+; sys_srecv() -> A = received byte. GB is the slave; blocks (yields) until the
+; peer clocks a byte in.
+sys_srecv::
+.poll
+    ld a, $80                   ; start transfer, external clock (receive)
+    ld [rSC], a
+    ld a, [rSC]
+    bit 7, a
+    jr z, .got                  ; bit7 clear => a byte arrived
+    call SchedYield
+    jr .poll
+.got
+    ld a, [rSB]
+    ret