Changes
adding SP-based method
==Using LDIR==
From the '''The Unofficial Amstrad WWW Resource'''
;;
;; etc....
</pre>
==Using the stack==
This method write bytes at a considerably faster rate by exploiting the speed of the stack pointer (SP) moving through RAM and its 2-byte steps. It is especially good for very rapidly clearing the screen.--[[User:Db6128|Db6128]] ([[User talk:Db6128|talk]]) 06:29, 13 December 2012 (EET)
<pre>
; Save the stack pointer
ld (SAVE_SP),sp ; Only remove if you're sure you don't need the stack
ld sp,FINAL_ADDRESS+1 ; e.g. to fill from &C000 to &FFFF (&4000 bytes), set SP to &FFFF+1 = &0000
; Define the region of RAM to be filled
ld h,YOUR_BYTE ; Use HL or whichever 16-bit register you prefer
ld l,YOUR_BYTE ; For 0, you can save 1 byte of memory by instead using XOR A:LD H,A:LD L,A
ld de,LENGTH_TO_FILL/2 ; divided by 2 because DE is 2 bytes wide and PUSH pushes both
; Set up a fast 16-bit loop counter
dec de ; This method takes advantage of the fact that DJNZ leaves B as 0, which subsequent DJNZs see as 256
ld b,e ; B = (length/2) MOD 256, so 0 = a 512-byte block
inc b ; D = the number of 512-byte blocks to write, or just = 1 if the length is <512
inc d ; Of course, if you know the length ahead of run-time, you can set B and D directly in your ASM
; Fill the memory
PUSHLOOP:
push hl ; Writes DE to (SP-2) and DECs SP by 2.
; For even more speed, use multiple PUSH HLs in a row (I like 8, = &10 bytes) and adjust counters to match
djnz PUSHLOOP
dec d
jr nz,PUSHLOOP
; Restore the stack pointer
SAVE_SP equ $+1
ld SP,&9999 ; &9999 will be replaced by the actual previous value
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[[Category:Programming]]
