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; FreeDiskSysROM

; Copyright (c) 2018 James Athey

;

; This program is free software: you can redistribute it and/or modify it under

; the terms of the GNU Lesser General Public License version 3 as published by

; the Free Software Foundation.

;

; This program is distributed in the hope that it will be useful, but WITHOUT

; ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS

; FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more

; details.

;

; You should have received a copy of the GNU Lesser General Public License

; along with this program. If not, see <https://www.gnu.org/licenses/>.

; This routine was likely planned to be used in order to avoid useless latency

; on VRAM reads. It compares the VRAM address in ($00) with the address in the

; Yth slot (minimum 1) of the read buffer. If both addresses match, the

; corresponding data byte is returned in A with the carry flag clear. If the

; addresses are different, the address in the Yth slot is overwritten with the

; address in ($00), and the carry flag is set on return.

; Parameters: X = starting index of read buffer, Y = read buffer slot # (minimum 1), $00, $01 = VRAM address to compare with address in read buffer slot

; Returns: carry clear : a previously read byte was returned, carry set : no byte was read, should wait next call to ReadVRAMBuffer

; Affects: A, X, Y

API_ENTRYPOINT $e94f

GetVRAMBufferByte:

DEX

DEX

DEX

TXA

@loop:

CLC

ADC #3

DEY

BNE @loop

TAX

LDA $00

CMP $300,X

BNE @UpdateAddressHi

LDA $01

CMP $301,X

BNE @UpdateAddressLo

LDA $302,X

CLC

RTS

@UpdateAddressHi:

STA $300,X

LDA $01

@UpdateAddressLo:

STA $301,X

SEC

RTS