78 lines
2.5 KiB
ArmAsm
78 lines
2.5 KiB
ArmAsm
;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
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;; Sets memory in 'A'-registry to all addresses from 'A_start' until 'A_start' + 'length'
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;; Modifies A, X and A_start
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.proc memset
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.include "mem.inc"
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;; big_set sets the memory in $ff chunks.
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;; skipp if length >= $ff
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LDX length +1
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BNE big_set
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JMP small_set
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big_set: ;sets $ff of memory
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;; Y value do not matter, will go through all anyway!
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.repeat $ff
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STA (A_start), Y
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DEY
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.endrepeat
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STA (A_start), Y ; dont forget Y =0
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big_set_end:
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;;set all hole $ff memory chunks!
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INC A_start + 1
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DEX ;; length +1 --
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BEQ small_set
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JMP big_set
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;; Note that cpu cykels total: cy_tot = 66 to 69
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;; But we skipp a BNE (cy = 2*) * [length (mod 255)]
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;; The BNE case has an avrige of 2*255/2 = 255 so this is faster (on avrige.)
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small_set:
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STA data_to_write ; cy = 3
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LDA length ; cy = 3
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STA length_copy ; cy = 3
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;; calculate rts-position
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LDX #$00 ; cy = 2
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STX length + 1 ; cy = 3
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;; 3 bytes = STA DEY NOP = seting 1 byte of memory.
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;; So we need to calculate: length*3
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Mult_16 A, length + 1 ; cy = 7
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; A= length
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ADC length_copy ; cy = 3
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TAY
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LDA length + 1 ; cy = 3
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ADC #$00 ; cy = 2
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STA length + 1 ; cy = 3
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;; Now RTS_pointer + Y = length*3 + big_set_label
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LDA #<big_set ; cy = 2
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STA RTS_pointer ; cy = 3
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LDA #>big_set ; cy = 2
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ADC length + 1 ; cy = 3
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STA RTS_pointer + 1 ; cy = 3
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;; read data we will change to RTS
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STY Y_copy ; cy = 3
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LDA (RTS_pointer), Y ; cy = 5*
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TAX ; cy = 2
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;; set RTS in big_set
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LDA #$60 ; cy = 2
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STA (RTS_pointer), Y ; cy = 5*
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;; JSR to modified big_set
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LDY length_copy ; cy = 3
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DEY ; because we want to count to Y=0 :)
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LDA data_to_write ; cy = 3
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JSR big_set ; cy = 6
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;; revert changes
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LDY Y_copy ; cy = 3
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TXA ; cy = 2
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STA (RTS_pointer), Y ; cy = 5*
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RTS
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.endproc
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