path: root/src/socket.asm

; =============================================================================
; socket.asm - the kernel network stack + socket layer (SYS_NET).
;
; The kernel owns the link port: SLIP framing, IPv4, checksums, and the per-
; protocol logic all live here. Userland just does socket()/connect()/send()/
; recv()/close() via one syscall (DE = &netreq). No program touches SLIP or IP.
;
; Our address is 10.0.0.2. Incoming ICMP echo requests are auto-answered while
; any process pumps RX (recv/poll), so the Game Boy responds to pings for real.
;
; Phase 1: ICMP (echo).  UDP and TCP build on the same frame/checksum core.
; =============================================================================
INCLUDE "include/gbos.inc"

; ---- socket entry layout ----------------------------------------------------
DEF SK_TYPE    EQU 0     ; 0 = free
DEF SK_RIP     EQU 1     ; 4  remote IP
DEF SK_RPORT   EQU 5     ; 2  remote port
DEF SK_LPORT   EQU 7     ; 2  local port
DEF SK_HASRX   EQU 9     ; 1  rxbuf holds a datagram
DEF SK_RXLEN   EQU 10    ; 1  payload length
DEF SK_SRCIP   EQU 11    ; 4  sender IP
DEF SK_SRCPORT EQU 15    ; 2  sender port
DEF SK_RXBUF   EQU 17    ; 96 payload
DEF SK_SIZE    EQU 113

SECTION "net_bss", WRAM0
wSockTab::     DS MAX_SOCKS * SK_SIZE
wNetOurIP::    DS 4
wNetReq::      DS NR_SIZE
wNetSockPtr::  DS 2
wNetIhl::      DS 1
wNetSeq::      DS 1
wNetTx::       DS 256      ; tx build buffer
wNetRxBuf::    DS 256      ; rx frame assembly / parse
wNetRxLen::    DS 1
wNetRxEsc::    DS 1
wNetUdpPort::  DS 2      ; scratch: UDP dst port being demuxed
wNetTO::       DS 3      ; recv timeout counter (net_pump clobbers registers)

SECTION "socket", ROM0

; net_init - clear the socket table and set our IP (called at boot).
net_init::
    ld hl, wSockTab
    ld bc, MAX_SOCKS * SK_SIZE
.clr
    xor a
    ld [hl+], a
    dec bc
    ld a, b
    or c
    jr nz, .clr
    ld a, 10
    ld [wNetOurIP+0], a
    xor a
    ld [wNetOurIP+1], a
    ld [wNetOurIP+2], a
    ld a, 2
    ld [wNetOurIP+3], a
    xor a
    ld [wNetRxLen], a
    ld [wNetRxEsc], a
    ld [wNetSeq], a
    ret

; -----------------------------------------------------------------------------
; low-level link port (byte at a time). Preserve BC/DE/HL; use A only.
net_putbyte:                    ; A = byte to transmit
    ld [rSB], a
    ld a, $81
    ld [rSC], a
.w
    ld a, [rSC]
    bit 7, a
    jr nz, .w
    ret

net_getbyte_nb:                 ; -> A = byte, CF set if none
    ld a, $80
    ld [rSC], a
    ld a, [rSC]
    bit 7, a
    jr z, .got
    scf
    ret
.got
    ld a, [rSB]
    and a                       ; CF = 0
    ret

; -----------------------------------------------------------------------------
; net_slip_send(HL = buf, B = len) - transmit a SLIP frame.
net_slip_send:
    ld a, SLIP_END
    call net_putbyte
.loop
    ld a, b
    or a
    jr z, .end
    ld a, [hl+]
    cp SLIP_END
    jr z, .esc_end
    cp SLIP_ESC
    jr z, .esc_esc
    call net_putbyte
.cont
    dec b
    jr .loop
.esc_end
    ld a, SLIP_ESC
    call net_putbyte
    ld a, SLIP_ESC_END
    call net_putbyte
    jr .cont
.esc_esc
    ld a, SLIP_ESC
    call net_putbyte
    ld a, SLIP_ESC_ESC
    call net_putbyte
    jr .cont
.end
    ld a, SLIP_END
    call net_putbyte
    ret

; -----------------------------------------------------------------------------
; net_sum(HL = buf, B = len, DE = seed) -> DE = folded 16-bit one's-complement
; sum (RFC 1071), carry-folded each add. len <= 255. Used to chain a pseudo-
; header into a segment checksum.
net_sum:
.loop
    ld a, b
    cp 2
    jr c, .tail
    ld a, [hl+]                 ; high byte
    ld c, a
    ld a, [hl+]                 ; low byte
    add e
    ld e, a
    ld a, c
    adc d
    ld d, a
    jr nc, .nc1
    inc de
.nc1
    dec b
    dec b
    jr .loop
.tail
    ld a, b
    or a
    ret z
    ld a, [hl]                  ; last odd byte -> high half
    add d
    ld d, a
    ret nc
    inc e
    ret nz
    inc d
    ret

; net_cksum(HL, B, DE=seed) -> HL = inverted 16-bit checksum.
net_cksum:
    call net_sum
    ld a, d
    cpl
    ld h, a
    ld a, e
    cpl
    ld l, a
    ret

; -----------------------------------------------------------------------------
; net_sock_ptr(A = sockid) -> HL = &wSockTab[A]  (clobbers B, DE)
net_sock_ptr:
    ld hl, wSockTab
    or a
    ret z
    ld b, a
    ld de, SK_SIZE
.l
    add hl, de
    dec b
    jr nz, .l
    ret

; =============================================================================
; sys_net(DE = &netreq) -> A = result
; =============================================================================
sys_net::
    ld hl, wNetReq              ; copy the request out of process RAM
    ld b, NR_SIZE
.cp
    ld a, [de]
    ld [hl+], a
    inc de
    dec b
    jr nz, .cp
    ld a, [wNetReq + NR_OP]
    cp NET_SOCKET
    jp z, net_op_socket
    cp NET_CONNECT
    jp z, net_op_connect
    cp NET_SEND
    jp z, net_op_send
    cp NET_RECV
    jp z, net_op_recv
    cp NET_CLOSE
    jp z, net_op_close
    cp NET_BIND
    jp z, net_op_bind
    cp NET_POLL
    jp z, net_op_poll
    ld a, $FF
    ret

; ---- bind(sock, port) - set local port -----------------------------------
net_op_bind:
    ld a, [wNetReq + NR_SOCK]
    call net_sock_ptr
    ld a, l
    add SK_LPORT
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [wNetReq + NR_PORT]
    ld [hl+], a
    ld a, [wNetReq + NR_PORT + 1]
    ld [hl], a
    xor a
    ret

; ---- socket(type) -> A = sockid or $FF ---------------------------------------
net_op_socket:
    ld c, 0
.scan
    ld a, c
    call net_sock_ptr           ; HL = &sock[c]
    ld a, [hl]
    or a                        ; SK_TYPE == 0 (free)?
    jr z, .free
    inc c
    ld a, c
    cp MAX_SOCKS
    jr c, .scan
    ld a, $FF                   ; table full
    ret
.free
    ld a, [wNetReq + NR_SOCK]   ; requested type
    ld [hl], a                  ; SK_TYPE
    ld a, l                     ; clear SK_HASRX
    add SK_HASRX
    ld l, a
    ld a, h
    adc 0
    ld h, a
    xor a
    ld [hl], a
    ld a, c                     ; return sockid
    ret

; ---- connect(sock, ip, port) - set remote --------------------------------
net_op_connect:
    ld a, [wNetReq + NR_SOCK]
    call net_sock_ptr           ; HL = &sock
    ld a, l
    add SK_RIP
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = &sock.RIP
    ld de, wNetReq + NR_IP
    ld b, 6                     ; 4 IP + 2 port (contiguous: RIP then RPORT)
.cp
    ld a, [de]
    ld [hl+], a
    inc de
    dec b
    jr nz, .cp
    xor a
    ret

; ---- close(sock) -------------------------------------------------------------
net_op_close:
    ld a, [wNetReq + NR_SOCK]
    call net_sock_ptr
    xor a
    ld [hl], a                  ; SK_TYPE = free
    ret

; ---- poll - pump RX once (answers pings) -------------------------------------
net_op_poll:
    call net_pump
    xor a
    ret

; ---- send(sock, buf, len) ----------------------------------------------------
net_op_send:
    ld a, [wNetReq + NR_SOCK]
    call net_sock_ptr
    ld a, l
    ld [wNetSockPtr], a
    ld a, h
    ld [wNetSockPtr+1], a
    ld a, [hl]                  ; SK_TYPE
    cp SOCK_ICMP
    jp z, icmp_send
    cp SOCK_UDP
    jp z, udp_send
    ld a, $FF
    ret

; ---- recv(sock, buf, maxlen) -> A = len or $FF (timeout) ---------------------
net_op_recv:
    ld a, [wNetReq + NR_SOCK]
    call net_sock_ptr
    ld a, l
    ld [wNetSockPtr], a
    ld a, h
    ld [wNetSockPtr+1], a
    xor a                       ; 24-bit timeout counter in WRAM (net_pump
    ld [wNetTO], a              ; clobbers BC/DE/HL, so we can't count in regs)
    ld [wNetTO+1], a
    ld [wNetTO+2], a
.wait
    call net_pump
    call sock_has_rx            ; nz if a datagram is buffered
    jr nz, .got
    ld hl, wNetTO
    inc [hl]
    jr nz, .wait                ; low byte didn't wrap -> spin (no yield)
    call SchedYield             ; every 256 pumps, yield to the scheduler
    inc hl
    inc [hl]
    jr nz, .wait
    inc hl
    inc [hl]
    ld a, [hl]
    cp 250                      ; timeout after ~250*65536 pumps
    jr c, .wait
    ld a, $FF
    ret
.got
    ; copy sock.RXBUF -> process NR_BUF, len = sock.RXLEN, NR_IP = sock.SRCIP
    call sock_ptr_hl            ; HL = &sock
    push hl
    ld a, l
    add SK_RXLEN
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [hl]                  ; RXLEN
    ld c, a                     ; C = length
    pop hl
    ; source IP -> wNetReq.NR_IP (returned to caller via buffer? no - copy to caller req is skipped; give src via NR_IP in kernel copy only)
    push bc
    push hl
    ; dst = process buf pointer
    ld a, [wNetReq + NR_BUF]
    ld e, a
    ld a, [wNetReq + NR_BUF + 1]
    ld d, a                     ; DE = process buf
    ld a, l
    add SK_RXBUF
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = &sock.RXBUF
    ld a, c
    or a
    jr z, .nocopy
.cpl
    ld a, [hl+]
    ld [de], a
    inc de
    dec c
    jr nz, .cpl
.nocopy
    pop hl                      ; &sock
    pop bc                      ; C = length
    ; clear HASRX
    ld a, l
    add SK_HASRX
    ld l, a
    ld a, h
    adc 0
    ld h, a
    xor a
    ld [hl], a
    ld a, c                     ; return length
    ret

; sock_ptr_hl -> HL = current socket pointer (from wNetSockPtr)
sock_ptr_hl:
    ld a, [wNetSockPtr]
    ld l, a
    ld a, [wNetSockPtr+1]
    ld h, a
    ret

; sock_has_rx -> Z clear (nz) if the current socket has a buffered datagram
sock_has_rx:
    call sock_ptr_hl
    ld a, l
    add SK_HASRX
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [hl]
    or a
    ret

; =============================================================================
; icmp_send - build + transmit an ICMP echo request from the current socket.
; =============================================================================
icmp_send:
    ; IP header (20 bytes)
    ld a, $45
    ld [wNetTx+0], a
    xor a
    ld [wNetTx+1], a
    ld [wNetTx+2], a            ; total len hi
    ld a, [wNetReq+NR_LEN]
    add 28
    ld [wNetTx+3], a            ; total len lo = 28 + payload
    xor a
    ld [wNetTx+4], a
    ld [wNetTx+5], a            ; id
    ld [wNetTx+6], a
    ld [wNetTx+7], a            ; flags/frag
    ld a, 64
    ld [wNetTx+8], a           ; ttl
    ld a, 1
    ld [wNetTx+9], a           ; proto ICMP
    xor a
    ld [wNetTx+10], a
    ld [wNetTx+11], a          ; ip cksum
    ld a, [wNetOurIP+0]
    ld [wNetTx+12], a
    ld a, [wNetOurIP+1]
    ld [wNetTx+13], a
    ld a, [wNetOurIP+2]
    ld [wNetTx+14], a
    ld a, [wNetOurIP+3]
    ld [wNetTx+15], a
    ; dst IP = sock.RIP
    call sock_ptr_hl
    ld a, l
    add SK_RIP
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = &sock.RIP
    ld de, wNetTx+16
    ld b, 4
.dip
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .dip
    ; ICMP header (8 bytes) at wNetTx+20
    ld a, 8
    ld [wNetTx+20], a          ; type = echo request
    xor a
    ld [wNetTx+21], a          ; code
    ld [wNetTx+22], a
    ld [wNetTx+23], a          ; cksum
    ld a, $12
    ld [wNetTx+24], a          ; id
    ld a, $34
    ld [wNetTx+25], a
    xor a
    ld [wNetTx+26], a          ; seq hi
    ld a, [wNetSeq]
    inc a
    ld [wNetSeq], a
    ld [wNetTx+27], a          ; seq lo
    ; payload from process buf
    ld a, [wNetReq+NR_BUF]
    ld l, a
    ld a, [wNetReq+NR_BUF+1]
    ld h, a
    ld de, wNetTx+28
    ld a, [wNetReq+NR_LEN]
    ld b, a
    or a
    jr z, .nopl
.cppl
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .cppl
.nopl
    ; ICMP checksum over 8 + payload
    ld a, [wNetReq+NR_LEN]
    add 8
    ld b, a
    ld hl, wNetTx+20
    ld de, 0
    call net_cksum
    ld a, h
    ld [wNetTx+22], a
    ld a, l
    ld [wNetTx+23], a
    ; IP checksum over 20
    ld hl, wNetTx
    ld b, 20
    ld de, 0
    call net_cksum
    ld a, h
    ld [wNetTx+10], a
    ld a, l
    ld [wNetTx+11], a
    ; transmit: 28 + payload
    ld a, [wNetReq+NR_LEN]
    add 28
    ld b, a
    ld hl, wNetTx
    call net_slip_send
    xor a
    ret

; =============================================================================
; udp_send - build + transmit a UDP datagram from the current socket.
; =============================================================================
udp_send:
    ld a, $45
    ld [wNetTx+0], a
    xor a
    ld [wNetTx+1], a
    ld [wNetTx+2], a
    ld a, [wNetReq+NR_LEN]
    add 28                      ; 20 IP + 8 UDP + payload
    ld [wNetTx+3], a
    xor a
    ld [wNetTx+4], a
    ld [wNetTx+5], a
    ld [wNetTx+6], a
    ld [wNetTx+7], a
    ld a, 64
    ld [wNetTx+8], a
    ld a, 17
    ld [wNetTx+9], a           ; proto UDP
    xor a
    ld [wNetTx+10], a
    ld [wNetTx+11], a
    ld a, [wNetOurIP+0]
    ld [wNetTx+12], a
    ld a, [wNetOurIP+1]
    ld [wNetTx+13], a
    ld a, [wNetOurIP+2]
    ld [wNetTx+14], a
    ld a, [wNetOurIP+3]
    ld [wNetTx+15], a
    ; dst IP = sock.RIP
    call sock_ptr_hl
    ld a, l
    add SK_RIP
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld de, wNetTx+16
    ld b, 4
.dip
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .dip
    ; UDP header at wNetTx+20: src port = SK_LPORT
    call sock_ptr_hl
    ld a, l
    add SK_LPORT
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [hl+]
    ld [wNetTx+20], a
    ld a, [hl]
    ld [wNetTx+21], a
    ; dst port = SK_RPORT
    call sock_ptr_hl
    ld a, l
    add SK_RPORT
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [hl+]
    ld [wNetTx+22], a
    ld a, [hl]
    ld [wNetTx+23], a
    ; length = 8 + payload
    xor a
    ld [wNetTx+24], a
    ld a, [wNetReq+NR_LEN]
    add 8
    ld [wNetTx+25], a
    xor a
    ld [wNetTx+26], a          ; cksum = 0
    ld [wNetTx+27], a
    ; payload from process buf
    ld a, [wNetReq+NR_BUF]
    ld l, a
    ld a, [wNetReq+NR_BUF+1]
    ld h, a
    ld de, wNetTx+28
    ld a, [wNetReq+NR_LEN]
    ld b, a
    or a
    jr z, .nopl
.cppl
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .cppl
.nopl
    ; UDP checksum with pseudo-header: sum(src+dst IP) + proto + udplen, then seg
    ld hl, wNetTx+12
    ld b, 8
    ld de, 0
    call net_sum               ; DE = IP-address sum
    ld a, e
    add 17                     ; + protocol
    ld e, a
    ld a, d
    adc 0
    ld d, a
    ld a, [wNetReq+NR_LEN]
    add 8                      ; + udp length
    add e
    ld e, a
    ld a, d
    adc 0
    ld d, a                    ; DE = pseudo-header seed
    ld a, [wNetReq+NR_LEN]
    add 8
    ld b, a
    ld hl, wNetTx+20
    call net_cksum             ; HL = inverted checksum
    ld a, h
    or l
    jr nz, .cknz
    ld hl, $FFFF               ; 0 means "no checksum" -> send all-ones
.cknz
    ld a, h
    ld [wNetTx+26], a
    ld a, l
    ld [wNetTx+27], a
    ; IP checksum
    ld hl, wNetTx
    ld b, 20
    ld de, 0
    call net_cksum
    ld a, h
    ld [wNetTx+10], a
    ld a, l
    ld [wNetTx+11], a
    ld a, [wNetReq+NR_LEN]
    add 28
    ld b, a
    ld hl, wNetTx
    call net_slip_send
    xor a
    ret

; =============================================================================
; net_pump - read available serial bytes, assemble SLIP frames, process each.
; Non-blocking: returns when the link has no more bytes ready.
; =============================================================================
net_pump:
.next
    call net_getbyte_nb
    ret c                       ; no byte ready
    ; SLIP decode using wNetRxLen / wNetRxEsc
    cp SLIP_END
    jr z, .frame
    ld c, a                     ; save byte
    ld a, [wNetRxEsc]
    or a
    jr nz, .escaped
    ld a, c
    cp SLIP_ESC
    jr z, .setesc
    jr .store
.escaped
    ld a, c
    cp SLIP_ESC_END
    jr nz, .notend
    ld c, SLIP_END
.notend
    ld a, c
    cp SLIP_ESC_ESC
    jr nz, .noteesc
    ld c, SLIP_ESC
.noteesc
    xor a
    ld [wNetRxEsc], a
    ld a, c
    jr .store2
.setesc
    ld a, 1
    ld [wNetRxEsc], a
    jr .next
.store
    ld a, c
.store2
    ; store A at wNetRxBuf[wNetRxLen] if room
    ld c, a
    ld a, [wNetRxLen]
    cp 255
    jr nc, .next                ; overflow: drop
    ld e, a
    ld d, 0
    ld hl, wNetRxBuf
    add hl, de
    ld a, c
    ld [hl], a
    ld a, [wNetRxLen]
    inc a
    ld [wNetRxLen], a
    jr .next
.frame
    ; END: if we have a frame, process it
    xor a
    ld [wNetRxEsc], a
    ld a, [wNetRxLen]
    or a
    jr z, .next                 ; empty frame
    push bc
    call process_frame
    pop bc
    xor a
    ld [wNetRxLen], a
    jr .next

; -----------------------------------------------------------------------------
; process_frame - IPv4 demux of the frame in wNetRxBuf (wNetRxLen bytes).
process_frame:
    ld a, [wNetRxBuf]
    and $F0
    cp $40
    ret nz                      ; not IPv4
    ld a, [wNetRxBuf]
    and $0F
    add a, a
    add a, a
    ld [wNetIhl], a             ; ihl in bytes
    ld a, [wNetRxBuf+9]         ; protocol
    cp 1
    jp z, icmp_in
    cp 17
    jp z, udp_in
    ret

; -----------------------------------------------------------------------------
; icmp_in - handle an inbound ICMP message.
icmp_in:
    ld a, [wNetIhl]
    ld e, a
    ld d, 0
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &icmp
    ld a, [hl]                  ; type
    cp 8
    jr z, icmp_request
    or a                        ; type 0 = echo reply
    jr z, icmp_reply
    ret

; incoming echo request -> answer it in place
icmp_request:
    ld a, [wNetIhl]
    ld e, a
    ld d, 0
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &icmp
    xor a
    ld [hl+], a                 ; type = 0
    inc hl                      ; skip code
    xor a
    ld [hl+], a
    ld [hl], a                  ; cksum = 0
    call net_swap_ip
    ; icmp cksum over (rxlen - ihl)
    ld a, [wNetIhl]
    ld e, a
    ld d, 0
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &icmp
    ld a, [wNetRxLen]
    sub e
    ld b, a
    ld de, 0
    push hl
    call net_cksum              ; HL = cksum
    pop de                      ; DE = &icmp
    ld a, e
    add 2
    ld e, a
    ld a, d
    adc 0
    ld d, a                     ; DE = &icmp+2
    ld a, h
    ld [de], a
    inc de
    ld a, l
    ld [de], a
    ; IP cksum
    xor a
    ld [wNetRxBuf+10], a
    ld [wNetRxBuf+11], a
    ld a, [wNetIhl]
    ld b, a
    ld hl, wNetRxBuf
    ld de, 0
    call net_cksum
    ld a, h
    ld [wNetRxBuf+10], a
    ld a, l
    ld [wNetRxBuf+11], a
    ; send it back
    ld a, [wNetRxLen]
    ld b, a
    ld hl, wNetRxBuf
    call net_slip_send
    ret

; incoming echo reply -> deliver to the first ICMP socket
icmp_reply:
    ld c, 0
.scan
    ld a, c
    call net_sock_ptr           ; HL = &sock
    ld a, [hl]
    cp SOCK_ICMP
    jr z, .deliver
    inc c
    ld a, c
    cp MAX_SOCKS
    jr c, .scan
    ret                         ; no ICMP socket
.deliver
    ; HL = &sock. payload = wNetRxBuf + ihl + 8, len = rxlen - ihl - 8
    push hl
    ; set HASRX = 1
    ld a, l
    add SK_HASRX
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, 1
    ld [hl], a
    pop hl
    ; SRCIP = wNetRxBuf[12..15]
    push hl
    ld a, l
    add SK_SRCIP
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld de, wNetRxBuf+12
    ld b, 4
.sip
    ld a, [de]
    ld [hl+], a
    inc de
    dec b
    jr nz, .sip
    pop hl
    ; RXLEN + RXBUF
    ld a, [wNetIhl]
    add 8
    ld e, a
    ld d, 0                     ; DE = ihl+8 (payload offset)
    ld a, [wNetRxLen]
    sub e
    ld c, a                     ; C = payload length
    push hl
    ld a, l
    add SK_RXLEN
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, c
    ld [hl], a                  ; RXLEN
    pop hl
    ; copy payload
    ld a, l
    add SK_RXBUF
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = &sock.RXBUF (dest)
    push hl
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &payload (src)
    pop de                      ; DE = dest
    ld a, c
    or a
    ret z
.cp
    ld a, [hl+]
    ld [de], a
    inc de
    dec c
    jr nz, .cp
    ret

; -----------------------------------------------------------------------------
; udp_in - deliver an inbound UDP datagram to the socket bound to its dst port.
udp_in:
    ld a, [wNetIhl]
    add 2
    ld e, a
    ld d, 0
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &udp.dstport
    ld a, [hl+]
    ld [wNetUdpPort], a
    ld a, [hl]
    ld [wNetUdpPort+1], a
    call net_find_udp           ; HL = &sock, CF if none
    ret c
    ; deliver: HASRX=1, SRCIP, RXLEN + RXBUF (payload after ihl+8)
    push hl
    ld a, l
    add SK_HASRX
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, 1
    ld [hl], a
    pop hl
    push hl
    ld a, l
    add SK_SRCIP
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld de, wNetRxBuf+12
    ld b, 4
.sip
    ld a, [de]
    ld [hl+], a
    inc de
    dec b
    jr nz, .sip
    pop hl
    ; payload offset = ihl + 8, length = rxlen - ihl - 8
    ld a, [wNetIhl]
    add 8
    ld e, a
    ld d, 0
    ld a, [wNetRxLen]
    sub e
    ld c, a                     ; C = payload length
    push hl
    ld a, l
    add SK_RXLEN
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, c
    ld [hl], a
    pop hl
    ld a, l
    add SK_RXBUF
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = &sock.RXBUF (dest)
    push hl
    ld hl, wNetRxBuf
    add hl, de                  ; HL = &payload (src)
    pop de                      ; DE = dest
    ld a, c
    or a
    ret z
.cp
    ld a, [hl+]
    ld [de], a
    inc de
    dec c
    jr nz, .cp
    ret

; net_find_udp -> HL = &UDP socket bound to wNetUdpPort, CF set if none
net_find_udp:
    ld c, 0
.l
    ld a, c
    call net_sock_ptr           ; HL = &sock (clobbers B, DE)
    ld a, [hl]
    cp SOCK_UDP
    jr nz, .nx
    push hl
    ld a, l
    add SK_LPORT
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [wNetUdpPort]
    cp [hl]
    jr nz, .nomatch
    inc hl
    ld a, [wNetUdpPort+1]
    cp [hl]
    jr nz, .nomatch
    pop hl                      ; match
    or a                        ; CF = 0
    ret
.nomatch
    pop hl
.nx
    inc c
    ld a, c
    cp MAX_SOCKS
    jr c, .l
    scf
    ret

; net_swap_ip - swap the src/dst IP fields of the frame in wNetRxBuf
net_swap_ip:
    ld hl, wNetRxBuf+12
    ld de, wNetRxBuf+16
    ld b, 4
.l
    ld a, [hl]
    ld c, a
    ld a, [de]
    ld [hl+], a
    ld a, c
    ld [de], a
    inc de
    dec b
    jr nz, .l
    ret