Raspberry Pi ARM ASM syscall notes

A few notes on useful syscalls:

SWI 0 : executes instruction based on value in R7

Write to screen: R7 = 4

MOV R7, #4 @ Syscall 4 = write to screen
MOV R0, #1 @ stdout: move 1 to R0
MOV R2, #4 @ length of string to R2
LDR R1, =string
    .ascii "testn"

Read from keyboard: R7 = 3

MOV R7, #3 @ Syscall 3 = read from keyboard
MOV R0, #0 @ stdin?: move 0 to R0
MOV R2, #4 @ length of string to ready to R2
LDR R1, =string @ load R1 value to string location 
    .ascii ""

Debugging JAXB unmarshalling issues

JAXB appears to fail silently in some cases if the XML it’s attempting to unmarshall to mapped classes doesn’t have the necessary mapped properties.

You can get additional output by adding the following:


– displays information during JAXB initialization

Before you call unmarshall() on your Unmarshaller, call setEventHandler() and add a DefaultValidationEventHandler as follows:

Unmarshaller um = jaxbContext.createUnmarshaller();
um.setEventHandler(new javax.xml.bind.helpers.DefaultValidationEventHandler());

– this will output additional failure information about missing mappings for xml elements, useful if your mapped class ends up with missing values.

Working split in FLDIGI

Most things are easy to work out in FLDIGI, but I just tried working a station that was working RTTY split, listening 1 up. You can certainly do this via settings on your radio but FLDIGI allows you to set different rx and tx freqs too – which is easier as it’s just point and click:

  • select the tx freq and press the ‘Lck’ button – this fixes the tx
  • clicking anywhere on the waterfall after you’ve pressed Lck now sets the receive

Don’t forget to unclick Lck when you’re done.

I got this from the manual here (funnily enough).