One of the tasks every BIOS vendors has to do when building a BIOS that sits on top of a Intel AMT enabled computer is to convert the text-mode display into a series of VT100 codes that are sent into the Intel AMT serial-over-LAN port to a terminal console. As I noticed when building IAmtTerm.exe , BIOS vendors do this very differently and sometimes very inefficiently.

 

The BIOS is in a position to capture displayed text when it's on the screen, something that the management engine (ME) on which Intel AMT runs can't do. Simply put, display and keyboard information does not flow thru Intel AMT. In order to provide text display and keyboard input remotely, Intel AMT provides the pipe (the virtual serial port) but needs the BIOS to do the rest. BIOS vendors where left to build code that would convert incoming VT100 codes into user key strokes and display changes into VT100 codes. As a result, how this operation is performed varies greatly from vendors to vendors, even for different BIOS from the same manufacturer (mobile vs. desktop).

 

 

The first difference is the handling of keys such as F1 to F10, there are no less than 3 different ways BIOS perform this conversion and so, IAmtTerm.exe provides 3 different mappings. Users have to change the key mapping manually and try them out. Secondly, some BIOS vendors tested against different VT100 terminals, notably terminals with only 24 lines instead of 25 lines. A real display and IAmtTerm.exe both have 25 lines, but many terminals have 24 since it was the convention during the modem days of the 80's to use the bottom text line as terminal status line. Some BIOS will try various tricks to display only 24 lines, hiding the top line, etc. Users that use IAmtTerm's "Terminal Analyzer" will also notice that different BIOS vendors convert text to VT100 differently. Some BIOS will send a full line whenever anything on that line has changed. Sometimes the codes for moving the cursor will be sent when the terminal cursor is already at that exact position on the screen. In one case, I even saw all of the display attributes (foreground and background color) be sent before each character on the screen, slowing down the display significantly.

 

 

In the latest versions of the Intel AMT Developer Tool Kit (DTK), I added Intel AMT Guardport, a light-weight C/C++ based serial agent. This new agent allows the user to shell to a command prompt and for the first time, Guardport also includes most of the same VT100 conversion that BIOS perform. When building Guardport, we took care to support all 3 F1 to F10 key mappings, and optimize VT100 display conversion to provide the best possible display speed. As a bonus, we also added proprietary mouse support, so you can enter a mouse enabled text application such as "Edit" and move the mouse over the application.

 

 

Ylian (Intel AMT Blog)