>�3:�23,1,1;0;0;0;:�14:�''"PRESS <SHIFT> TO SCROLL"'':�text
O�''"PLEASE PRESS <P> TO PRINT TEXT, <L> TO LOAD PROGRAM, <M> FOR MAIN MENU"
�:� �="P" � �="L" � �="M"
(� �="M" � �"$.MENU"
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d��"U S I N G   T H E   A S S E M B L E R   L A N G U A G E - PART SEVEN"'"********************************************************************"'
nM�"Post-Index (full name Post-Indexed Indirect) addressing is one form of"
xQ�"assembly addressing,  Page 207 of the handbook has a paragraph devoted  to"
�P�"Post Index.  Looking back at Part Five a line was added to  PROCconstants"
�O�"to declare address &70 as ""temp"" and at line 207 there is  LDA(temp),Y"
�L�"that is a Post-Index statement.  Post-Index may only use a  zero page"
�M�"address and the Y register.   The electron needs to locate a  two byte"
�I�"address, so that it may load data stored at that address into  the"
�N�"accumulator.  Targetting or pointing to an exact location in memory  is"
�Q�"""addressing"", ""Him over there"", ""Her as should know better"" are also"
�P�"forms of addressing but the electron not being blessed with human  powers"
�L�"of deduction needs a specific instruction in a predetermined  format."
��
�P�"In our orginal program the label ""data"" is an address, if the statement"
�O�"LDA data MOD256:STAtemp were to be written, RUN and then CALLed the low" 
�S�"byte of data would be stored in &70 which had been declared as ""temp"",  if"

P�"a further statement LDA data DIV256:STAtemp+1 had also recieved the  same"

O�"treatment the high byte of ""data"" would be stored in &70+1 = &71   the"

-�"capital Y is of course the Y register."

"�

,R�"Imagine that ""data"" was the label for address &2000 and that the value in"

6N�"the Y register was four, then  temp (&70) would hold the value &00 and" 

@N�"temp+1 (&71) would hold the value &20 the statement LDA (temp),Y would" 

JQ�"load the accumulator with the content of address &2000+4 = &2004.  If  the"

TL�"value in the Y register were to be decimal 12 the content of address" 

^2�"&200C would be loaded into the accumulator."

h�

rM�"Why not just load the low byte of data into &70 and the high byte into"

|Q�"&71 direct and index it to Y.  The post index statement would then be  LDA"

�Q�"(&70),Y.  No reason except that zero page locations are in short  supply." 

�O�"To designate two of them as temp and use them only as temporary  storage"

�O�"areas, prevents having to look back over lines of source code  to find a"

�P�"free zero page address.  Why use this method at all?  The  answer to that"

�S�"is ""Our address is on the stack in two bytes"".  The  orginal ""data"" is a"

�P�"label and a label may be indexed with either the X  or Y register.  Using"

�P�"PROCprt a single multi-user print routine, and  jumping to it with JSRprt"

�R�"causes ""data"" to be stored on the stack in  two bytes.  That is ideal for"

�J�"Post-Index just pull the values and store  them in temp and temp+1."

��

�R�"Imagine that a program requires blocks of data to be stored or  retrieved." 

�Q�"Then a table for storing those addresses is very useful and  in some cases"

��"totally necessary."
�
O�"To store the address in the table, the low byte would be placed in temp" 
Q�"and the high byte in temp+1 and in the same way the high and low bytes  of"
&Q�"table would need to be stored in two more zero page addresses say  loc and"
0�"loc+1 (location)"
:�
DM�"Then the statement LDA(temp),Y:STA(loc),Y:INY:LDA(temp),Y:STA(loc),Y"  
NM�"would place the address into successive bytes in the table.  Consider" 
XQ�"that situation, by manipulating the value in the Y register ten  addresses"
bJ�"with a common high byte could be stored in eleven bytes  instead of"
lO�"twenty.  The high byte would be stored  at the base of the  table, after"
vN�"retrieving the high byte with Y set to zero Y would then  be set to the"
�L�"offset for the particular low byte required (a value  between two and"
�*�"eleven) and the low byte retrieved."
��
�K�"Take that further if the blocks of data were stable and did not move"
�O�"about in memory then the high byte need not be stored at all.  The zero" 
�?�"page temp+1 could be loaded absolute with the high byte" 
�L�"LDA#&20:STAtemp+1.  Not elegant coding but needs must when the devil" 
�8�"wills.  Thirty kilobyte of user RAM is not a lot."
��
�P�"To sum up the syntax of LDA(temp),Y activates a routine in the 6502 chip" 
�K�"that adds the low byte of an address to the high byte then adds the" 
�E�"content of the Y register to give a final sixteen bit address."
��
K�"Reverse colour text window?  How did it go? My code is listed below."
K�"Load the program and alter it to my listing, PROCconstants has been" 
O�"altered slightly, merely to give us more control of the program. PROCprt"
 �"no changes."
*�
4Q�"PROCdata has altered the labels indicate the action of the code. Line  380"
>O�"clears the screen with a VDU12.  Colours are reversed at Line 390  where"
HN�"EQUD is introduced, four EQUBs might have been used EQUB17:EQUB0  would"
RJ�"have given a black foreground and EQUB17:EQUB129 white  background."
\�
fM�"Remember the Acorn routine at &FFEE3 reads and understands the control"
p*�"codes (Appendix F of the handbook)."
z�
�O�"EQUD is neater and saves some typing but both EQUD and EQUW only accept" 
�N�"hexidecimal numbers, worse they insist that the numbers are in reverse" 
�Q�"order.  At first sight line 390 is very confusing but divide it into  four"
�S�"bytes, (use a scrap of paper) ""&81""  ""&11"" ""&00"" ""&11"" now ask  your"
�O�"computer PRINT &81 the answer is 129 write it under the &81 do the  same"
�W�"with the others.  Answer ""129"" ""17"" ""0"" ""17"" reading from right to  left"
�V�"(Backwards) ""17"" ""0"" (black foreground) ""17"" ""129"" (white  background)."
��
�Q�"Line 400 is different, the text window needs five bytes, the last byte  of"
�Q�"the EQUD statement is &1C 16+12=28 (VDU28) the next the left hand  edge of"
�Q�"the window ""0"" then the bottom edge ""2""  (0,1,2) 0 is a number  to the"
�S�"computer so ""2"" gives three lines deep. &27 2x16+7=39 sets the  right hand"
�P�"edge. Tucked on the end is an EQUB0 which sets the top edge.   410 clears"
P�"the screen within the text window.  420 EQUW&081F; &1F  equals 31 (VDU31)"
N�"&08 for column eight and the EQUB1 line number 1.  line 440 returns the"
M�"colours to standard and 450 window off. 460 tabs  the cursor to column"
$=�"zero line fifteen and the routine concludes with  &FF."
.�
8Q�"EQUB, EQUW and EQUD are all used for inserting numbers into assembly  code"
BL�"they are basic statements that may only be used in assembly  language"
LM�"source code.  EQUB inserts a single byte, EQUW a word (two  bytes) and"
V/�"EQUD inserts a double word (four bytes)."
`�
jM�"Next time start%, base% and scrn1 will be used to save the object code"
tD�"plus if space permits an OSBYTE call to the operating system."
~M�"**********************************************************************"
�M�"PLEASE NOTE: THE LISTING IS SAVED AS A PROGRAM ON THE DISC AS $.ASSEM2"
�N�"**********************************************************************" 
��
�

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