XA User Guide 6-8 4/17/98
6.3 Relative Branching and Jumps
Program memory addresses as referenced by Jumps, Calls, and Branch instructions must be word
aligned in XA. For instance, a branch instruction may occur at any code address, but it may only
branch to an even address. This forced alignment to even address provides three benefits:
• Branch ranges are doubled without providing an extra bit in the instruction and
• Faster execution as XA always fetches first two byte of an instruction simultaneously.
• Allows translated 8051 code to have branches extended over intervening code that will tend to
grow when translated and generally increase the chances of a branch target being in that
range.
The rel8 displacement is a 9-bit two’s complement integer which is encoded as 8-bits that
represents the relative distance in words from the current PC to the destination PC. Similarly, the
rel16 displacement is a 17-bit twos complement integer which is encoded as 16-bits. The value of
the PC used in the target address calculation is the address of the instruction following the Branch,
Jump or Call instruction.
The 8-bit signed displacement is between -128 to +127. The branch range for rel8 is (sample
calculation shown below) is really +254 bytes to -256 bytes for instructions located at an even
address, and +253 to -257 for the same located at an odd address, with the limitation that the target
address is word aligned in code memory.
The 16-bit signed displacement is -32,768 to +32,767. The branch range is therefore +65,534 bytes
to -65,536 bytes for instructions located at an even address, and +65,533 to -65,537 for the same
located at an odd address, with the limitation that the target address is word aligned in code
memory.
Sample calculation for rel8 range:
Assuming word aligned branch target, forward range as measured from current PC is:
Branch Target Address - Current PC
Now, maximum positive signed 8-bit displacement = +127; So, rel8 << 1 is +254
If Current PC = ODD, then
Range = 254 - 1 = +253 as PC is forced to an even location, else
If current PC = EVEN, then
Range = +254
Similarly, reverse range as measured from current PC is:
Branch Target Address - Current PC
Now, maximum positive signed 8-bit displacement = -128; So, rel8 << 1 is -256
If Current PC = ODD, then
Range = -257
Else if current PC = EVEN, then
Range = -256
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