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Section
05 Part 06 The BSR Instruction |
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Some parents say it is toy guns that make boys warlike. But
give a boy a rubber duck and he will seize its neck like the butt of a pistol
and shout 'Bang! ~George Will |
Introduction
BSR Branch to SubRoutine
The destination operand is added to the PC, the
return address is moved into the stack, the 68k will continue reading at the
new offset held in PC.
Examples
This
is similar to JSR with a few minor differences (In fact, the same differences
between BRA and JMP). Heres a small list
as a quick example:
|
bsr.s Subroutine jmp Continue Subroutine: rts Continue: ...etc |
Now
obviously:
That
part you should know from reading Section 05 Part 05 (JSR and RTS).
Once
again, the main difference between BSR and JSR, is memory and processing
time. The BSR instruction is similar to
BRA in that you have two different sizes .s for short, and .w for word.
For bsr.s, the add value is only a byte big, and it is a
signed value. This means the maximum
number of bytes it can branch forwards by is 7E, and the maximum it can branch backwards by is 80. So
obviously bsr.s cannot reach certain places where
the JSR instruction can.
For bsr.w, the add value here is a word big and is also
signed. This now means the maximum
number of bytes it can branch forwards by is 7FFE, and the maximum it can branch backwards by is 8000.
As
you can see, the range is larger than bsr.s, but
still smaller than the JSR instruction.
Usage
Just
like BRA and JMP, with BSR and JSR youll be looking to use the quickest and
smallest instructions you possibly can, below is a table to help narrow out the
relationship between the two branch sizes and the jump:
|
Instruction |
Memory/Size |
Range |
|
|
|
|
|
bsr.s |
61
?? |
7E bytes
forwards or backwards |
|
bsr.w |
61
00 ?? ?? |
7FFE
bytes forwards or backwards |
|
jsr |
4E
B9 00 ?? ?? ?? |
000000
FFFFFF (Unlimited) |
Your
priority of course is:
|
bsr.s -> bsr.w
-> jsr |