Thanks SamMeEl ,Hamada-san,Bob,Yanagisawa-san for replying.
But I still can't make it work.

My environment:
PowerBook 550C, Japanese Keyboard, No internal modem, 12MB memory
Mac OS 7.6 English, IBM 750MB SCSI HDD, almost dead internal backup battery.

Kernel: HWDIRECT-SBC#61

Boot still stops at 'zstty0' line most of the times even if I tick the 
Booter option 'Serial Port->Open before...' on. But sometimes it just boots
fine.

I tried to boot more than 50 times and I'm getting convinced that this
is a Booter problem in that it fails to open serial port. Is it possible to 
verify this?  I can step down to debugger when boot stops,

zstty0 at zsc0 at channel 0
Panic switch: PC is 0x117de0.
Stopped at     _Debugger+....
db> trace
..
_lev7intr(?)
_zs_iflush(...)
_zs_loadchannelregs(...)
..


Boot always stops here when it stops.

Yanagisawa-san suggested a possible workaround.  To build a kernel without
serial port driver.  Is this easily doable?



 > Date: Tue, 3 Mar 1998 03:30:06 -0800 (PST)
 > From: SamMaEl <rimsky@teleport.com>
 > 
 > On Tue, 3 Mar 1998, INOUE Katsumi wrote:
 > > Is anyone using 1.3 on PowerBook 550? I have tried 6 kinds of kernels 
 > > and never successfully booted. I have tried all the Booter options. Also
 > > I have tried with extensions off.


--
Katsumi INOUE