img0059.txt

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FILE: tromdS:EOS_TF          HEWLETT-PACKARD:   A_uOS 00   (c) Coleco  {iS-.Confidential                   Sat,     8  Sep  1984,  23:39        .aGE     55

LOCATION  OBJECT CODE   LINE      SOURCE  LINE

                                 Name:             SET_READ  / SET WRITE

                                 Function:         Initiates   the VDP for reads  or writes.

                                 Entry:            HL  - VRAM address  to start  read / writes

                                 Exit:             C register  contains  the VDP_DATA_PORT  vatue
                                                   HL remains  unchanged

                                 Registers  used:  AF,  BC, HL,

                                 Data  structures  accessed:

                                                   PORT_TABLE  (CTRL_PORT_PTR,  DATA_PORT PTR)   for
                        2103  ;                    VDP  port access.

                                 Size:             ROM  - xx bytes
                              ;                    RAM  - xx bytes

                                 Comments:

                                 Compar ision:

    E1E7                2112  SSET_READ:
    E1E7  AF            2113      .       XOR   A                 ;Clear accumulator  for later OR  which
                        2114                                      ; Getermines a  read or write

    E1E8  C2            2116              DEFB  OC2H              :This is the beginning  of a JP  NZ which
                        2117                                      ; will never be  true but leaves  the
                        2118                                      ; accumulator  intact over the  next
                        2119                                      ; two bytes (  the LD A,40H  is decoded
                        2120                                      ; as the last  two bytes of the  jump
    E1E9                2121  SETWRITE:
    E1E9  3&40          2122              LD    A,40H             ;Set bit to tell  VDP that write  is requested

    E1EB  ED4BFC29      2124              LO    BC,[VDP_CTRL_PORT]   ;Get BOTH port  addresses
    E1EF  ED69          2125              OUT   [C].L             ;Output low 8 bits  of address
    E1Fi  84            2126              OR   4H                 ;OR read/write  bit (Bit 6) into  the high    8 bytes
    E1F2  ED79          2127              OuT   [CJ.A             ;Output result
    E1F4  48            2128              iD    C,B               ;Put DATA_PORT  value into the  C register

    E1FS  C9            2130              RET                     ;Exit to caller