The most likely interpretation is that the 2-bit field will have a value
of "01", with a decimal value of 1. This is what DFDL calls
dfdl:bitOrder="mostSignificantBitFirst" and is what most data formats use.
DFDL also has dfdl:bitOrder="leastSignificantBitFirst", which is common
in some old military formats like VMF and Link16. One way to imagine
this is as if the memory addresses and bits were ordered and read
right-to-left. So, your example would look like this:
High memory address Low memory address
0 0 ................................................ 1 1 0
So we have the same data, and you still read the low memory address bits
first, it's just all backwards. Because we read right-to-left, the first
two bits are now "10". Note that interpreting the value of the bits is
the same as normal, so "10" evaluates to 2--all that changes is the
order in which we read bits.
I haven't seen any formats where the high memory address bits would be
read first and would lead to the 2-bit field being "00". DFDL doesn't
have a way to model this.
On 2023-12-07 12:28 PM, Roger L Costello wrote:
Hi Folks,
A basic question about bits.
Scenario: an application is receiving a message. The message arrives as a
stream of bits. The first bit received is 0. The second bit received is 1. The
third bit received is another 1. ... The second-to-last bit received is 0. The
last bit received is 0.
The application stores the message in memory.
Will the first bit received be in the lowest memory address and the last bit
received in the highest memory address? I.e.,
Low memory address High memory address
0 1 1 ............................................................. 0 0
The message contains a series of bit fields. The specification for the message
says the first field is two bits.
Is the first field the bits 0 1 or the bits 00?
/Roger
