small CBDOS refactoring

src/dos_m65/filesystem_cbm/fs_cbm_nextfileblock.s

@@ -0,0 +1,101 @@
+
+;
+; CBM/CMD filesystem - helper routine to get the next file block
+;
+
+
+fs_cbm_nextfileblock:
+
+	; Fetch the next track/sector
+
+	lda PAR_SECTOR
+	and #$01
+	bne @part_2
+
+@part_1:
+
+	; We were reading from the 1st half of the buffer
+
+	ldx SHARED_BUF_1+1
+	lda SHARED_BUF_1+0
+	bra @common_chk
+
+@part_2:
+
+	; We were reading from the 2nd half of the buffer
+
+	ldx SHARED_BUF_1+$100+1
+	lda SHARED_BUF_1+$100+0
+
+@common_chk:
+
+	bne @common_load
+
+	lda #$00                           ; end of file
+	sec
+	rts
+
+@common_load:
+
+	sta PAR_TRACK
+	stx PAR_SECTOR
+
+	; FALLTROUGH
+
+fs_cbm_nextfileblock_got_ts:
+
+	; Load next track/sector
+
+	jsr lowlevel_readsector         ; XXX handle read errors
+
+	; Store pointer and amount of data ready to fetch
+
+	lda #$FE
+	sta FD_ACPTR_LEN+0
+	lda #$00
+	sta FD_ACPTR_LEN+1
+
+	lda #$02
+	sta FD_ACPTR_PTR+0
+	lda #>SHARED_BUF_1
+	sta FD_ACPTR_PTR+1
+
+	lda PAR_SECTOR
+	and #$01
+	bne @part_2
+
+@part_1:
+
+	; Sector with data is located in the 1st half of the buffer
+
+	lda SHARED_BUF_1+0
+	beq @exit
+
+	; Less than a full sector is used
+
+	lda SHARED_BUF_1+1
+	bra @not_full_buf
+
+@part_2:
+
+	; Sector with data is located in the 2nd half of the buffer
+
+	inc FD_ACPTR_PTR+1
+
+	lda SHARED_BUF_1+$100+0
+	beq @exit
+
+	; Less than a full sector is used
+
+	lda SHARED_BUF_1+$100+0
+
+@not_full_buf:
+
+	sta FD_ACPTR_LEN+0
+
+	; FALLTROUGH
+
+@exit:
+
+	clc
+	rts