; =============================================================================
; 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