About the start address and length of "_DATA_GROUP_"

A little addendum.

A serial ISR would probably manage with just two 8-bit registers for it's work (plus what is needed for a return address and to protect processor flags).

Either to check the status of the serial port, to compare the read/write positions of a send or receive ring buffer, to pick up one character from a transmit buffer, move the read position one step and put the character in the serial transmit register or pick up one character from the serial receive register put in the receive buffer and step the write position one step.

If the OP feels the need to allocate large areas of memory in the serial ISR, it is quite obvious that a non-optimum solution has been selected.

A little addendum.

A serial ISR would probably manage with just two 8-bit registers for it's work (plus what is needed for a return address and to protect processor flags).

Either to check the status of the serial port, to compare the read/write positions of a send or receive ring buffer, to pick up one character from a transmit buffer, move the read position one step and put the character in the serial transmit register or pick up one character from the serial receive register put in the receive buffer and step the write position one step.

If the OP feels the need to allocate large areas of memory in the serial ISR, it is quite obvious that a non-optimum solution has been selected.