# Backward link zero

Backward link zero (BLZ) is a particular system error.
A frame containing data, either workspace or file system data was
not properly linked to a previous frame of data. Instead, this backward
pointer was zero. The system was traversing data backward and ran
off the beginning of the data.

Each frame of data has both forward and backward links.
If the frame has a zero backward link it is usually the first frame
detailing some information or document. Any frame with a backward
link can be chased through the backward chain of frames to find the
first frame (beginning of the data).

When at TCL, use dump with the lu options to travel backward
through links. When in the system debugger use the l command to see the links of a frame and press CTRL+P to travel backward
one link.

```
Backward Link Zero ; reg = {number} abort @ {abs mode}
```

## Example(s)

Pressing CTRL+P at the `=` (equal sign) displays the link fields for the previous
frame 740219 and typing CTRL+N displays the link fields for the next
frame 740221.

```
debug
I ut.go.debug:07E
! l740221
!l740220 740220 0 : 740221 740219 : 0 =
```

## See also

- [BLZ](https://d3codex.com/definitions/blz/)
- [dump command](https://d3codex.com/tcl/dump-command/)

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Source: https://d3codex.com/definitions/backward-link-zero/ - part of the D3Codex reference.