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+; =============================================================================
+; 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