Elan eSL Series, User Manual

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Chapter 2
eSL/eSLS Series (+ eSLZ000) User’s Manual Architecture
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2.5.6 Control Instructions
The instructions in this group are used for the program control flow. CALL,
RET and RETI instructions provided subroutine and interrupt execution.
Unconditional overhead free program loop constructs are supported using the
RPT and LOOP instructions.
CALL: Before jumping to target address, the 16-bit return address (PC+1) is
pushed into the stack.
RET: Pop the return address to PC then return from subroutine.
RETI: Pop the return address to PC then return from interrupt service
routine.
RPT: Repeat the next instruction
LOOP:
Zero-overhead LOOP (must include at least 3 instruction; but the
last instruction cannot be JMP, CALL, RETURN or RPT
instruction. See Section 2.5.6.2 below).
The Repeat (RPT) function may be tied in with such instructions as
Multiply/Accumulate (MAC) and Block Moves (MOV) to increase execution
speed of RPT instruction. These multicycle instructions effectively become
single-cycle instructions after the first iteration of a repeat instruction.
2.5.6.1 Operating Instruction Example Syntax
Mnemonic Example Syntax
Original Cycle
*
MOV [R5] = P[R7++] 2
MUL.UU D = R2 * P[R3++] (UU) 2
MUL.US D = R5 * P[R7++] (US) 2
MUL.SU D = R3 * [R5 ++] (SU) 1
MUL.SS D = R4 * P[R3++] (SS) 2
MAC D = D + R2 * P[R1++] 2
MAS D = D – R2 * P[R6++] 2
ADD R1 = [R2] + R3 1
SUB R3 = [R2] - R1 1
*
Number of cycles when instruction is not repeated
2.5.6.2 RPT and LOOP Instructions Limitations
Some instructions cannot be repeated with RPT instruction and cannot be the
last instruction in a LOOP. These instructions are as below.
Mnemonic Description
CALL Unconditional call
JMP Instructions
Branch instruction
(Include unconditional or condition brach)
RET Return from subroutine
RETI Return from interrupt
RPT Repeat next instruction