Add ScrAddress library

Author: boriel

docs/library/scraddress.md

@@ -0,0 +1,62 @@
+#ScrAddress
+
+This function returns the address in screen memory of the TOP line of the character in print position X-Y. 
+Remember that the next line will be 256 bytes further on, and the 3rd line 256 further again and so forth,
+for 7 more lines.
+
+```
+FUNCTION scrAddress(x as uByte, y as uByte) as Uinteger
+asm 
+; This function returns the address into HL of the screen address
+; x,y in character grid notation. 
+; Original code was extracted by BloodBaz
+     ; x Arrives in A, y is in stack.
+     and     31
+     ld      l,a
+     ld      a,(IX+7) ; Y value
+     ld      d,a
+     and     24
+     add     a,64
+     ld      h,a
+     ld      a,d
+     and     7
+     rrca
+     rrca
+     rrca
+     or      l
+     ld      l,a
+end asm
+END FUNCTION
+
+
+FUNCTION attrAddress (x as uByte, y as uByte) as uInteger               
+';; This function returns the memory address of the Character Position
+';; x,y in the attribute screen memory.
+';; Adapted from code by Jonathan Cauldwell.
+
+asm
+     ld      a,(IX+7)        ;ypos
+     rrca
+     rrca
+     rrca               ; Multiply by 32
+     ld      l,a        ; Pass to L
+     and     3          ; Mask with 00000011
+     add     a,88       ; 88 * 256 = 22528 - start of attributes.
+     ld      h,a        ; Put it in the High Byte
+     ld      a,l        ; We get y value *32
+     and     224        ; Mask with 11100000
+     ld      l,a        ; Put it in L
+     ld      a,(IX+5)   ; xpos 
+     add     a,l        ; Add it to the Low byte
+     ld      l,a        ; Put it back in L, and we're done. HL=Address.
+end asm
+END FUNCTION
+```
+
+Examples of use (though more likely to be used as parameters to other screen handling functions):
+
+
+```
+Print scrAddress(8,15)
+Print attrAddress(8,15)
+```