petviewer.asm

pethead	.text "PET"	;Magic
		.text "1"	;Version
		
valid		.text "PET1"

petviewer ;RegPtr -> Pointer to a fileref to a PET image
		.block
ptr		= $fb

		lda #ff_r
		jsr fopen
		
		;Read in the Magic and Version
		jsr fread
		.word pethead
		.word 4

		#stxy ptr

		;Validate the Magic and Version
		ldx #3
nextv		lda pethead,x
		cmp valid,x
		bne invalid
		dex
		bpl nextv

		;Read 53 more header bytes to /dev/null
		ldy #freflfn
		lda (ptr),y
		tax
		jsr chkin
		
		ldx #53
nextr		jsr chrin
		dex
		bne nextr

		jsr clrchn

		#rdxy ptr

		;Read in screen memory
		jsr fread
		.word scrmem
		.word 1000
		
		;Read in color memory
		jsr read
		.word colmem
		.word 1000
		
		;Read VIC background/border registers
		jsr fread
		.word vic+$20
		.word 2
		
invalid	jsr fclose
		
		ldx #1
		jmp pgfree
		.bend

Updated the file flags, I'd been going from memory.