; make primary header
mphdr:
; HL - where to store 6-byte header
; DE - source sequence count
; BC - packet length
; A  - ApID less 0x700
        push psw
        push b
        push d
        push h

; Byte 0
;  3 bits of version number = 000, fixed                                      
;  1 bit of type = 0, fixed                                             
;  1 bit showing presence of secondary header = 1, fixed                        
;  3 bits which are the MSB's of the APID = 111, fixed
;    (all ISUAL ApIDs are 7XX)
        mvi M,0Fh
        inx h

; Byte 1
;  8 bits which are the LSB's of the APID.                                    
        mov M,a
        inx h

; Byte 2
;  2 bits = 11, fixed, which show that segmentation is not being used   
;  6 bits which are the MSB's of the source sequence count (variable)        
        mov a,d         ; MSB of source seq count
        ani 03Fh        ; mask to 14 bits
        ori 0C0h        ; merge in '11'
        mov M,a
        inx h

; Byte 3
;   8 bits which are the LSB's of the source sequence count  (variable)        
        mov M,e         ; LSB of source seq count
        inx h

; Length of the Packet Data Field                                   
;  portion of the packet.
; The value includes the Secondary Header 
;  and the Trailer, minus 1.
;  BC=Total_packet_length
        xchg
        lxi h,-7   ;  deduct length of primary hdr +1
        dad b

; Byte 4
;   8 bits which are the MSB's of the data length
        xchg
        mov M,d
        inx h

; Byte 5
;   8 bits which are the LSB's of the data length.  
        mov M,e

        pop h
        pop d
        pop b
        pop psw
        ret