~ National ~ Semiconductor - Al Kossow's Bitsavers

6.0 Instruction Set Reference (Continued)
RET Unconditional Return
Syntax
RET
{g (,rf))
Affected Flags
If rf= 1 then N, Z, C, and V
Description
Unconditionally returns control to the last instruction address
pushed onto the internal Address Stack by popping
that address into the Program Counter. The unconditional
return instruction also has two optional operands, g and rf.
The value of g determines if the Global Interrupt Enable bit
[GIE] is left unchanged (g=O), restored from the Address
Stack (g= 1), set (g=2), or cleared (g=3). If the g operand
is omitted then g = 0 is assumed. The second optional operand,
rf, determines if the ALU flags and register bank selections
are left unchanged (rf=O), or restored from the Address
Stack (rf= 1). If the rf operand is omitted then rf=O is
assumed.
Example
Return from an interrupt.
RET 1,1 ;Restore environment & return
Instruction Format
11 I 0 I 1 I ~~;o~~ I 1 I 1 11 I g I rf 1 0 I 0 I 0 I 0 1
15 6! 4 3 0
T-states
2
Bus Timing
Figure 7
Operation
OO-GIE not affected
01-Restore GIE
10-SetGIE
11-Clear GIE
Case g of
0: leave [GIE] unaffected, (default)
1: restore [GIE] from Address Stack
2: set [GIE]
3: clear [GIE]
End case
If rf= 1 then
restore ALU flags from Address Stack
restore register bank selection from Address Stack
End if
Address Stack ---+ PC
ROT
Syntax
ROT
Rotate
Rsd, b
Affected Flags
N,Z,C
Description
-register
Rotates the contents of the register Rsd b bits to the right
and places the result back into that register. The bits that
are shifted out of the LSB are shifted back into the MSB,
(and copied into the Carry flag). The value b may specify
from 0 to 7 bit rotates.
Example
Add 3 to the Address Stack Pointer contained in the Internal
Stack Pointer register (iSP), R30.
MOVE ISP, R8 ;get (iSP)
ROT R8, 4 ;shift [ASP] to low order nibble
ADD 3, R8 ;add 3 to [ASP]
ROT R8, 4 ;shift [ASP] to high order nibble
MOVE R8, ISP ;store new (iSP)
Instruction Format
15 7
T-states
2
Bus Timing
Figure 7
Operation
Rsd
I I
Rsd
4 o
TL/F9336-12
•
2-119