I'm just preparing on this for my MCT tomorrow.
I'll try to explain as I was figuring this out
The temperature data register format from(reading temperature)/to(setting TH and/or TL for thermostat use) DS1620 is the following:

MSB LSB
|X|X|X|X|X|X|X|1| |0|1|0|0|1|1|0|1|
X- 7 MSB bits are being ignored
1 in MSB means that reading is negative (0 would be positive)
Last bit of LSB (which is 1) means that there's .5 to the reading
so here's what we get
-|01001101|,5 => 01001101(negative bin) = 10110010+1(positive bin) => 10110011(postive bin) => 179(dec)
-|01001101|,5 = -|179|,5 => -179,5
you won't be quite getting anything like this as ds1620 can only do -55c to +125c