Hi my name is ***** ***** I will be the expert working with you today to help and resolve the problem you have posted.
I am sorry to hear that you are having such trouble. I will try my best to help you out and resolve the problem as quick as possible.
Since how long are you facing this problem?
is it windows 8 you have an an operating system on the computer?
open devices and printers and right click on the printer canon mx870 and select printer properties.
Let me know once you are there.
now click on the testprint button and let me know if it prints half the page or its printing full test page?
looks like a driver corruption issue.
Power off the printer and unplug the usb.
Uninstall printer driver completely from the computer and reboot the pc.
let me give you the download link then you can install it back.
sure let me know how it goes.
run the print head cleaning and allignment 2 times that should fix the problem.
what you might try is checking the ink cartridges to see if they have a little vent to let air in to replace the ink that was expended
if those get clogged than the ink stops flowing. usually in a few seconds it will get enough air in to print again. But by that time the page is finished. so it starts printing the next page and runs into the same problem halfway down.
if you can find the vents you can clean them with a pin.
Or just try a new cartridge. It is unlikely for it to be this problem with every color. but if you have been printing pages all or mostly one color such as black it might not be noticeable that it is only one color.
you can try making test pages of one color or another to see if the problem repeats itself.
Another possibility is that you have set the printer or software to print on a smaller page than you are using. In this case the second half of the page will show up on the next page or the back of the page if you have an auto duplexer.