Use old offset calculation, save ptr3

Looks like cprintf modifies ptr3, so make sure to save it if you are
calling that.

There also may have been an issue with how the previous sector address
was calculated, the one which does not account for carries seems to
work.

sw/bios/boot2.s

@@ -101,6 +101,11 @@ _start:
         ldx #>word_str
         jsr pushax
 
+        lda ptr3
+        pha
+        lda ptr3 + 1
+        pha
+
         ldy #$1d                ; load file size (256)
         lda (ptr3),y
         lsr                     ; divide by 2 to get file size (512)
@@ -108,26 +113,29 @@ _start:
         ldy #$4
         jsr _cprintf
 
+        pla
+        sta ptr3 + 1
+        pla
+        sta ptr3
+
         ldy #$1b                ; load high byte of low first cluster
         lda (ptr3),y
         tax
         dey
         lda (ptr3),y            ; load low byte of low first cluster
 
-        ldx data_start + 1
         sec
-        sbc #$02
-        bcs @3
-        dex
-@3:     asl
+        sbc #$02                ; don't handle carry, assume low byte is not 0 or 1
+        ldx data_start + 1      ; load x as high data start
+        asl                     ; multiply cluster num (minus 2) by 8
         asl
         asl
         clc
-        adc data_start
-        bcc @5
+        adc data_start          ; add that to low data start
+        bcc @5                  ; handle carry
         inx
-@5:     stz sreg                ; data start is not going to be > 2^16
-        stz sreg + 1            ; (I hope)
+@5:     stz sreg
+        stz sreg+1
         phx
         pha