Tech Arsenal 1/Tech Arsenal (Arsenal Computer).ISO/tek-05/driverss.zip/DEPCA.ASM

Assembly Source File | 1991-01-27 | 19KB | 828 lines

version    equ    0

    include    defs.asm

NI_CSR        equ    0
RBI        equ    2
DATA_REG    equ    4
ADDR_REG    equ    6
    CSR0        equ    0
    CSR1        equ    1
    CSR2        equ    2
    CSR3        equ    3
EBASE        equ    0ch

outport    macro    reg
    push    ax
    setport    ADDR_REG
    mov    ax,reg
    out    dx,ax
    in    ax,dx            ;always follow a write by a read

    setport    DATA_REG
    pop    ax
    out    dx,ax
    in    ax,dx            ;always follow a write by a read

    endm


;
;     Control and Status Register 0 (CSR0) bit definitions
;
CSR0_ERR    equ     8000h    ; Error summary
CSR0_BABL    equ     4000h    ; Babble transmitter timeout error
CSR0_CERR    equ    2000h    ; Collision Error
CSR0_MISS    equ    1000h    ; Missed packet
CSR0_MERR    equ    0800h    ; Memory Error
CSR0_RINT    equ    0400h    ; Reciever Interrupt
CSR0_TINT       equ    0200h    ; Transmit Interrupt
CSR0_IDON    equ    0100h    ; Initialization Done
CSR0_INTR    equ    0080h    ; Interrupt Flag
CSR0_INEA    equ    0040h    ; Interrupt Enable
CSR0_RXON    equ    0020h    ; Receiver on
CSR0_TXON    equ    0010h   ; Transmitter on
CSR0_TDMD    equ    0008h    ; Transmit Demand
CSR0_STOP    equ    0004h     ; Stop
CSR0_STRT    equ    0002h    ; Start
CSR0_INIT    equ    0001h    ; Initialize

;
;     Initialization Block  Mode operation Bit Definitions.
;
M_PROM        equ    8000h    ; Promiscuous Mode
M_INTL        equ    0040h   ; Internal Loopback
M_DRTY        equ    0020h   ; Disable Retry
M_COLL        equ    0010h    ; Force Collision
M_DTCR        equ    0008h    ; Disable Transmit CRC)
M_LOOP        equ    0004h    ; Loopback
M_DTX        equ    0002h    ; Disable the Transmitter
M_DRX        equ    0001h   ; Disable the Reciever


;
;     Receive message descriptor bit definitions.
;
RCV_OWN        equ    8000h    ; owner bit 0 = host, 1 = lance
RCV_ERR        equ    4000h    ; Error Summary
RCV_FRAM    equ     2000h    ; Framing Error
RCV_OFLO    equ    1000h    ; Overflow Error
RCV_CRC        equ    0800h    ; CRC Error
RCV_BUF_ERR    equ     0400h    ; Buffer Error
RCV_START    equ    0200h    ; Start of Packet
RCV_END        equ    0100h    ; End of Packet


;
;    Transmit  message descriptor bit definitions.
;
XMIT_OWN    equ    8000h    ; owner bit 0 = host, 1 = lance
XMIT_ERR    equ    4000h   ; Error Summary
XMIT_RETRY    equ    1000h   ; more the 1 retry needed to Xmit
XMIT_1_RETRY    equ    0800h    ; one retry needed to Xmit
XMIT_DEF    equ    0400h    ; Deferred
XMIT_START    equ    0200h    ; Start of Packet
XMIT_END    equ    0100h    ; End of Packet

;
;    Miscellaneous Equates
;

TRANSMIT_BUF_COUNT    equ    1
RECEIVE_BUF_COUNT    equ    8
TRANSMIT_BUF_SIZE    equ    1518
RECEIVE_BUF_SIZE    equ    1518

;
;    Receive Message Descriptor
;
rcv_msg_dscp struc
    rmd0    dw    ?    ; Rec. Buffer Lo-Address
    rmd1    dw    ?    ; Status bits / Hi-Address
    rmd2    dw    ?    ; Buff Byte-length (2's Comp)
    rmd3    dw    ?    ; Receive message length
rcv_msg_dscp ends


;
;    Transmit Message Descriptor
;
xmit_msg_dscp struc
    tmd0    dw    ?    ; Xmit Buffer Lo-Address
    tmd1    dw    ?    ; Status bits / Hi-Address
    tmd2     dw    ?    ; Buff Byte-length (2's Comp)
    tmd3    dw    ?    ; Buffer Status bits & TDR value
xmit_msg_dscp ends

lance_seg    segment at 0

;
;the LANCE requires that the descriptor pointers be on a qword boundary.
;
    align    8

transmit_dscps    xmit_msg_dscp    TRANSMIT_BUF_COUNT dup(<>)
receive_dscps    rcv_msg_dscp    RECEIVE_BUF_COUNT dup(<>)

;
;      LANCE Initialization Block
;
    align    2
init_block        label    byte
init_mode        dw    0
init_addr        db    EADDR_LEN dup(?)    ; Our Ethernet address
init_filter        db    8 dup(0)    ;Multicast filter.
init_receive        dw    ?,?        ;Receive Ring Pointer.
init_transmit          dw    ?,?          ;Transmit Ring Pointer.

transmit_bufs    equ    $
receive_bufs    equ    transmit_bufs + TRANSMIT_BUF_COUNT * TRANSMIT_BUF_SIZE

lance_seg    ends

code    segment    para public
    assume    cs:code, ds:code

        public    int_no
int_no        db    2,0,0,0            ;must be four bytes long for get_number.
io_addr        dw    -1,-1
base_addr    dw    -1,-1
depca_pattern    db    0FFh, 00h, 55h, 0AAh, 0FFh, 00h, 55h, 0AAh

    public    driver_class, driver_type, driver_name, driver_function, parameter_list
driver_class    db    BLUEBOOK, IEEE8023, 0        ;from the packet spec
driver_type    db    66        ;from the packet spec
driver_name    db    'DEPCA',0    ;name of the driver.
driver_function    db    2        ;basic, extended
parameter_list    label    byte
    db    1    ;major rev of packet driver
    db    9    ;minor rev of packet driver
    db    14    ;length of parameter list
    db    EADDR_LEN    ;length of MAC-layer address
    dw    GIANT    ;MTU, including MAC headers
    dw    MAX_MULTICAST * EADDR_LEN    ;buffer size of multicast addrs
    dw    RECEIVE_BUF_COUNT-1    ;(# of back-to-back MTU rcvs) - 1
    dw    TRANSMIT_BUF_COUNT-1    ;(# of successive xmits) - 1
int_num    dw    0    ;Interrupt # to hook for post-EOI
            ;processing, 0 == none,

    public    rcv_modes
rcv_modes    dw    7        ;number of receive modes in our table.
        dw    0               ;There is no mode zero
        dw    rcv_mode_1
        dw    0        ;only ours.
        dw    rcv_mode_3    ;ours plus broadcast
        dw    0        ;some multicasts
        dw    rcv_mode_5    ;all multicasts
        dw    rcv_mode_6    ;all packets

transmit_head    dw    transmit_dscps    ;->next packet to be filled by host.
receive_head    dw    receive_dscps    ;->next packet to be filled by LANCE.

    public    as_send_pkt
; The Asynchronous Transmit Packet routine.
; Enter with es:di -> i/o control block, ds:si -> packet, cx = packet length,
;   interrupts possibly enabled.
; Exit with nc if ok, or else cy if error, dh set to error number.
;   es:di and interrupt enable flag preserved on exit.
as_send_pkt:
    ret

    public    drop_pkt
; Drop a packet from the queue.
; Enter with es:di -> iocb.
drop_pkt:
    assume    ds:nothing
    ret

    public    xmit
; Process a transmit interrupt with the least possible latency to achieve
;   back-to-back packet transmissions.
; May only use ax and dx.
xmit:
    assume    ds:nothing
    ret


    public    send_pkt
send_pkt:
;enter with ds:si -> packet, cx = packet length.
;exit with nc if ok, or else cy if error, dh set to error number.
    assume    ds:nothing
    mov    es,base_addr
    assume    es:lance_seg

    xor    bx,bx

    mov    ax,18
    call    set_timeout
send_pkt_1:
    test    transmit_dscps[bx].tmd1,XMIT_OWN    ;Did the lance chip give it back?
    je    send_pkt_2
    call    do_timeout
    jne    send_pkt_1
    mov    dh,CANT_SEND
    stc
    ret
send_pkt_2:
;reset error indications.
    and    transmit_dscps[bx].tmd1,not (XMIT_ERR or XMIT_DEF or XMIT_1_RETRY or XMIT_RETRY)    ;Did the lance chip give it back?
    mov    transmit_dscps[bx].tmd3,0    ;reset all error bits.

    mov    ax,cx            ;store the count.
    cmp    ax,RUNT            ; minimum length for Ether
    ja    oklen
    mov    ax,RUNT            ; make sure size at least RUNT
oklen:
    neg    ax
    mov    transmit_dscps[bx].tmd2,ax    ;store the negative of the cnt.

    mov    ax,transmit_dscps[bx].tmd0    ;store the packet.
    mov    dx,transmit_dscps[bx].tmd1
    call    phys_to_segmoffs
    assume    es:nothing
    rep    movsb

    mov    es,base_addr
    assume    es:lance_seg
    or    transmit_dscps[bx].tmd1,XMIT_OWN    ;give it to the lance chip.

;Inform LANCE that it should poll for a packet.
    loadport
    mov    ax,CSR0_INEA or CSR0_TDMD
    outport    CSR0
    clc
    ret


detect_board:
;test to see if a board is located at io_addr.
;setup to read first byte of ethernet address rom when successful.
;return nz if not.
    assume    cs:code, ds:code

    loadport
    setport    EBASE
    mov    si, offset depca_pattern
    lodsb                ;get first char of test pattern
    mov    ah,al            ;move string byte to ah
    mov    di,0
    mov    cx,32            ;do 32 reps
detect_board_1:
    in    al,dx            ;input byte
    cmp    ah,al
    jne    detect_board_2
    lodsb                ;get next char
    mov    ah,al
    inc    di
    cmp    di, 8            ;need to match eight chars
    je    detect_board_done
detect_board_2:
    loop    detect_board_1
    or    sp,sp            ;guaranteed to be nonzero.
detect_board_done:
    ret


    public    get_address
get_address:
;get the address of the interface.
;enter with es:di -> place to get the address, cx = size of address buffer.
;exit with nc, cx = actual size of address, or cy if buffer not big enough.
    assume    ds:code, es:nothing
    cmp    cx,EADDR_LEN        ;make sure that we have enough room.
    jb    get_address_2
    push    di
    call    detect_board
    pop    di
    mov    cx,EADDR_LEN
    loadport            ; Get our Ethernet address base.
    setport    EBASE+1
    cld
get_address_1:
    in    al,dx            ; get a byte of the eprom address
    stosb
    loop    get_address_1        ; go back for rest
    mov    cx,EADDR_LEN
    clc
    ret
get_address_2:
    stc
    ret


    public    set_address
set_address:
;enter with ds:si -> Ethernet address, CX = length of address.
;exit with nc if okay, or cy, dh=error if any errors.
    assume    ds:nothing
    cmp    cx,EADDR_LEN        ;ensure that their address is okay.
    je    set_address_4
    mov    dh,BAD_ADDRESS
    stc
    jmp    short set_address_done
set_address_4:

    mov    es,base_addr
    mov    di,offset init_addr
    rep    movsb
    call    initialize        ;initialize with our new address.

set_address_okay:
    mov    cx,EADDR_LEN        ;return their address length.
    clc
set_address_done:
    push    cs
    pop    ds
    assume    ds:code
    ret


rcv_mode_1:
    mov    ax,M_DRX or M_DTX    ;disable the receiver and transmitter.
    jmp    initialize_nomulti
rcv_mode_3:
    xor    ax,ax            ;don't accept any multicast frames.
    call    initialize_multi
    mov    ax,0            ;non-promiscuous mode
    jmp    short initialize_nomulti
rcv_mode_5:
    mov    ax,-1            ;accept any multicast frames.
    call    initialize_multi
    mov    ax,0            ;non-promiscuous mode
    jmp    short initialize_nomulti
rcv_mode_6:
    mov    ax,M_PROM    ;promiscuous mode
initialize_nomulti:
initialize:
    mov    es,base_addr
    mov    es:init_mode,ax

    loadport
    mov    ax,CSR0_STOP        ;reset the INIT bit.
    outport    CSR0

    mov    ax,CSR0_INEA or CSR0_STRT or CSR0_INIT    ;reinit and restart.
    outport    CSR0

    setport    DATA_REG

    mov    ax,36            ;wait one second for the board
    call    set_timeout        ;  to timeout.
initialize_1:
    in    ax,dx
    test    ax,CSR0_IDON
    jne    initialize_2
    call    do_timeout
    jne    initialize_1
    stc
    ret
initialize_2:
    clc
    ret


initialize_multi:
;enter with ax = value for all multicast hash bits.
    mov    es,base_addr
    mov    di,offset init_filter
    mov    cx,8/2
    rep    stosw
    ret


    public    set_multicast_list
set_multicast_list:
;enter with ds:si ->list of multicast addresses, cx = number of addresses.
;return nc if we set all of them, or cy,dh=error if we didn't.
    mov    dh,NO_MULTICAST        ;for some reason we can't do multi's.
    stc
    ret


    public    terminate
terminate:
    call    rcv_mode_1        ;don't receive any apckets.

    ret

    public    reset_interface
reset_interface:
;reset the interface.
    assume    ds:code
    ret


;called when we want to determine what to do with a received packet.
;enter with cx = packet length, es:di -> packet type, dl = packet class.
    extrn    recv_find: near

;called after we have copied the packet into the buffer.
;enter with ds:si ->the packet, cx = length of the packet.
    extrn    recv_copy: near

    extrn    count_in_err: near
    extrn    count_out_err: near

LANCE_ISR_ACKNOWLEDGE equ (CSR0_INEA or CSR0_TDMD or CSR0_STOP or CSR0_STRT or CSR0_INIT)

    public    recv
recv:
;called from the recv isr.  All registers have been saved, and ds=cs.
;Upon exit, the interrupt will be acknowledged.
    assume    ds:code

    loadport
    setport    ADDR_REG
    mov    ax,CSR0
    out    dx,ax
    in    ax,dx
    setport    DATA_REG
    in    ax,dx
    mov    bx,ax            ;make a copy.

; Acknowledge the Interrupt from the controller, but disable further
; controller Interrupts until we service the current interrupt.
;
;(CSR0_INEA or CSR0_TDMD or CSR0_STOP or CSR0_STRT or CSR0_INIT)
;
    and    ax,not LANCE_ISR_ACKNOWLEDGE
    out    dx,ax
    in    ax,dx        ; follow all writes by a read

    test    bx,CSR0_RINT        ;receive interrupt?
    jne    recv_RINT        ;yes.
    jmp    recv_done        ;no, we're done.
recv_RINT:

    mov    es,base_addr
    assume    es:lance_seg
    mov    bx,receive_head

recv_search:
    test    lance_seg:[bx].rmd1,RCV_OWN    ;do we own this buffer?
    je    recv_own        ;yes - process it.
    call    inc_recv_ring        ;go to the next one.
    cmp    bx,receive_head        ;did we get back to the beginning?
    jne    recv_search        ;not yet.
    jmp    recv_done        ;yes -- spurious interrupt!
recv_own:
    test    lance_seg:[bx].rmd1,RCV_ERR    ;Any errors in this buffer?
    jne    recv_err        ;yes -- ignore this packet.

    mov    cx,lance_seg:[bx].rmd3
    and    cx,0fffh        ;strip off the reserved bits

    mov    ax,lance_seg:[bx].rmd0    ;fetch the packet.
    mov    dx,lance_seg:[bx].rmd1
    call    phys_to_segmoffs
    assume    es:nothing

    push    es
    push    di
    push    bx

    add    di,EADDR_LEN+EADDR_LEN    ;skip the ethernet addreses and
                    ;  point to the packet type.
    mov    dl, BLUEBOOK        ;assume bluebook Ethernet.
    mov    ax, es:[di]
    xchg    ah, al
    cmp     ax, 1500
    ja    BlueBookPacket
    inc    di            ;set di to 802.2 header
    inc    di
    mov    dl, IEEE8023
BlueBookPacket:
    push    cx
    call    recv_find
    pop    cx

    pop    bx
    pop    si
    pop    ds
    assume    ds:nothing, es:nothing

    mov    ax,es            ;is this pointer null?
    or    ax,di
    je    recv_free        ;yes - just free the frame.

    push    es
    push    di
    push    cx
    rep    movsb
    pop    cx
    pop    si
    pop    ds
    assume    ds:nothing

    call    recv_copy

    jmp    short recv_free

recv_err:
    call    count_in_err
recv_free:
    push    cs
    pop    ds
    assume    ds:code
    mov    es,base_addr
    assume    es:lance_seg

;clear any error bits.
    and    lance_seg:[bx].rmd1,not (RCV_ERR or RCV_FRAM or RCV_OFLO or RCV_CRC or RCV_BUF_ERR)
    or    lance_seg:[bx].rmd1,RCV_OWN    ;give it back to the lance.
    call    inc_recv_ring            ;go to the next one.
    test    lance_seg:[bx].rmd1,RCV_OWN    ;Do we own this one?
    je    recv_own
    mov    receive_head,bx        ;remember where the next one starts.
recv_done:
    loadport            ;enable interrupts again.
    setport    DATA_REG
    mov    ax,CSR0_INEA
    out    dx,ax

    ret


inc_recv_ring:
;advance bx to the next receive ring descriptor.
    assume    ds:nothing
    add    bx,(size rcv_msg_dscp)
    cmp    bx,offset receive_dscps + RECEIVE_BUF_COUNT * (size rcv_msg_dscp)
    jb    inc_recv_ring_1
    mov    bx,offset receive_dscps
inc_recv_ring_1:
    ret


    public    recv_exiting
recv_exiting:
;called from the recv isr after interrupts have been acknowledged.
;Only ds and ax have been saved.
    assume    ds:nothing
    ret


phys_to_segmoffs:
;enter with dx:ax as the physical address of the buffer,
;exit with es:di -> buffer.
    assume    ds:nothing
  if 0
    shl    dx,16-4            ;move the upper four bits into position.
    mov    di,ax            ;now get the low 12 bits of the segment.
    shr    di,4
    or    dx,di            ;combine them.
    mov    es,dx
    mov    di,ax
    and    di,0fh            ;now compute the offset.
  else
    mov    es,base_addr
    mov    di,ax
  endif
    ret

    include    timeout.asm

end_resident    label    byte

    public    usage_msg
usage_msg    db    "usage: depca [-n] [-d] [-w] <packet_int_no> <int_no> <io_addr> <mem_addr>",CR,LF,'$'
no_board_msg    db    "No DEPCA detected.",CR,LF,'$'
io_addr_funny_msg    label    byte
        db    "No DEPCA detected, continuing anyway.",CR,LF,'$'
bad_reset_msg    db    "Unable to reset the DEPCA.",CR,LF,'$'
bad_init_msg    db    "Unable to initialize the DEPCA.",CR,LF,'$'

    public    copyright_msg
copyright_msg    db    "Packet driver for a Digital Equipment Corporation DEPCA, version ",'0'+majver,".",'0'+version,CR,LF
        db    '$'

int_no_name    db    "Interrupt number ",'$'
io_addr_name    db    "I/O port ",'$'
base_addr_name    db    "Memory address ",'$'

    extrn    set_recv_isr: near
    extrn    maskint: near

;enter with si -> argument string, di -> dword to store.
;if there is no number, don't change the number.
    extrn    get_number: near

;enter with si -> argument string, di -> dword to print.
    extrn    print_number: near

    public    parse_args
parse_args:
;exit with nc if all went well, cy otherwise.
    assume    ds:code
    mov    di,offset int_no
    call    get_number
    mov    di,offset io_addr
    call    get_number
    mov    di,offset base_addr
    call    get_number
    clc
    ret


    public    etopen
etopen:
    assume    ds:code

    cmp    io_addr,-1        ;Did they ask for auto-detect?
    je    find_board

    call    detect_board        ;no, just verify its existance.
    je    find_board_found

    mov    dx,offset io_addr_funny_msg
    mov    ah,9
    int    21h

    jmp    find_board_found

find_board:
    mov    io_addr,300h        ;Search for the Ethernet address at 300h
    mov    io_addr+2,0
    call    detect_board
    je    find_board_found
    mov    io_addr,200h        ;Search at 200h
    call    detect_board
    je    find_board_found

    mov    dx,offset no_board_msg    ;Tell them that we can't find it.
    mov    ah,9
    int    21h

    stc
    ret
find_board_found:

    mov    al, int_no        ; Get board's interrupt vector
    add    al, 8
    cmp    al, 8+8            ; Is it a slave 8259 interrupt?
    jb    set_int_num        ; No.
    add    al, 70h - 8 - 8        ; Map it to the real interrupt.
set_int_num:
    xor    ah, ah            ; Clear high byte
    mov    int_num, ax        ; Set parameter_list int num.

    mov    al,int_no
    call    maskint            ;disable these interrupts.

    loadport
    mov    ax,CSR0_STOP        ;reset the INIT bit.
    outport    CSR0

;set up transmit descriptor ring.
    mov    es,base_addr
    assume    es:lance_seg
    mov    cx,TRANSMIT_BUF_COUNT
    mov    bx,offset transmit_dscps
    mov    di,offset transmit_bufs
setup_transmit:
    call    segmoffs_to_phys

    or    dx,XMIT_START or XMIT_END
    mov    lance_seg:[bx].tmd0,ax        ;points to the buffer.
    mov    lance_seg:[bx].tmd1,dx

    add    bx,(size xmit_msg_dscp)
    add    di,TRANSMIT_BUF_SIZE
    loop    setup_transmit

;set up receive descriptor ring.
    mov    cx,RECEIVE_BUF_COUNT
    mov    bx,offset receive_dscps
    mov    di,offset receive_bufs
setup_receive:
    call    segmoffs_to_phys

    or    dx,RCV_OWN
    mov    lance_seg:[bx].rmd0,ax        ;points to the buffer.
    mov    lance_seg:[bx].rmd1,dx

    mov    lance_seg:[bx].rmd2,-RECEIVE_BUF_SIZE
    mov    lance_seg:[bx].rmd3,0

    add    bx,(size rcv_msg_dscp)
    add    di,RECEIVE_BUF_SIZE
    loop    setup_receive

;initialize the board.
    mov    cx,EADDR_LEN        ;get our address.
    mov    di,offset init_addr
    call    get_address

    mov    cx,RECEIVE_BUF_COUNT
    call    compute_log2

    mov    di,offset receive_dscps
    call    segmoffs_to_phys
    or    dx,cx            ;include the buffer size bits.
    mov    init_receive[0],ax
    mov    init_receive[2],dx

    mov    cx,TRANSMIT_BUF_COUNT
    call    compute_log2

    mov    di,offset transmit_dscps
    call    segmoffs_to_phys
    or    dx,cx            ;include the buffer size bits.
    mov    init_transmit[0],ax
    mov    init_transmit[2],dx

    mov    di,offset init_block    ;now tell the board where the init
    call    segmoffs_to_phys    ;  block is.

    push    dx
    push    ax

    loadport
    mov    ax,2            ;write the bus config register.
    outport    CSR3

    pop    ax            ;write the low word.
    outport    CSR1

    pop    ax            ;write the high word.
    outport    CSR2

    xor    ax,ax            ;don't accept any multicast frames.
    call    initialize_multi
    mov    ax,0            ;non-promiscuous mode
    call    initialize
    jnc    init_ok

    mov    dx,offset bad_init_msg
    mov    ah,9
    int    21h

    stc
    ret

init_ok:
;
; Now hook in our interrupt
;
    call    set_recv_isr

    loadport
    setport    NI_CSR
    mov    al,82h            ;disable ROM, enable the interrupt line
    out    dx,al

    mov    dx,offset end_resident
    clc
    ret

    public    print_parameters
print_parameters:
;echo our command-line parameters
    mov    di,offset int_no
    mov    dx,offset int_no_name
    call    print_number
    mov    di,offset io_addr
    mov    dx,offset io_addr_name
    call    print_number
    mov    di,offset base_addr
    mov    dx,offset base_addr_name
    call    print_number
    ret

compute_log2:
;enter with cx = number of buffers.
;exit with cx = log2(number of buffers) << 13.
    mov    ax,-1
compute_log2_1:
    inc    ax
    shr    cx,1
    jne    compute_log2_1
    mov    cl,13
    shl    ax,cl
    mov    cx,ax
    ret


segmoffs_to_phys:
;enter with es:di -> buffer.
;exit with dx:ax as the physical address of the buffer,

  if 0    ; The DEPCA doesn't use system memory, it uses its own.
    mov    dx,es            ;get the high 4 bits of the segment,
    shr    dx,16-4
    mov    ax,es            ;and the low 12 bits of the segment.
    shl    ax,4
    add    ax,di            ;add in the offset.
    adc    dx,0
  else
    xor    dx,dx            ;the offset is the only part of the
    mov    ax,di            ;  address.
  endif
    ret


code    ends

    end