Code Example: REP
potatohead
Posts: 10,261
Here is another one. We may reverse the operands. That has not happened yet, but the label based syntax has. I came up with "FallThrough" as a way to get at that label ambiguity. Feel free to post up bugs, errors, whatever. I'll make the edits, and get a GitHUB doc started in the near future here.
If you have one you want to do, I suggest just using the same format. These are searchable in the forum easy.
REP - Repeat sequence of instructions as zero overhead loop
If you have one you want to do, I suggest just using the same format. These are searchable in the forum easy.
REP - Repeat sequence of instructions as zero overhead loop
CCCC 1100110 1LI DDDDDDDDD SSSSSSSSS REP D/#,S/#
CCCC - Condition Codes
Opcode = %1101011
D = Value sourced from Destination COG address
S = Value sourced from Source COG address
L = Destination Address mode: 0 = Value sourced from COG Destination Register
1 = Immediate "#", value sourced from D instruction bits
I = Source Address mode: 0 = Value sourced from COG Source Register
1 = Immediate "#", value sourced from S instruction bits
# = D as immediate value $0-$1ff
## = works with AUGD, AUGS for large immediate values $0-$7F_FFFF****
AUGD #$7F_FFFF —> target D
AUGS #$7F_FFFF --->target S
target REP #D, #S
Examples:
Assembler will calculate proper values based on label:
REP #@.FallThrough, #6
shl D, S
add D, S
.FallThrough JMP #elsewhere
The SHL and ADD instructions are executed 6 times, including the first time through the REP loop. When REP is complete, it falls through to the next instruction identified by "@label" as shown with ".FallThrough" above.
Immediate Values, you write:
start REP #1, ##10000 'Assembler will insert AUGS at label “start”
shl D, S ‘Instruction 0
add D, S ‘Instruction 1, for a total of 2
JMP #elsewhere
PASM that results:
start AUGS #10000 ’23 bit immediate value —> modify REP
modify REP #1, #S ‘value comes from AUGS
shl D, S ‘Instruction 0
add D, S ‘Instruction 1, for a total of 2
JMP #elsewhere
The SHL and ADD instructions get executed 10000 times, including the first time through the REP loop. When REP is complete, it falls through to the next instruction, JMP as shown above.
When computing the number of instructions to be repeated, start your count from 0. Two instructions are shown above, for a count total of 1. (number of instructions - 1)
Mixed Values:
REP #@.FallThrough, count
shl D, S
add D, S
.FallThrough JMP #elsewhere
[…]
count long 5
The SHL and ADD instructions get executed 5 times, including the first time through the REP loop. The COG register at label count contains the number of times REP instructions get executed. When REP is complete, it falls through to the next instruction identified by "@label" as shown with ".FallThrough" above.
When a label is used, the assembler will calculate the correct value needed to include all the instructions BETWEEN the REP instruction and the fall through instruction, to be executed after REP has finished.
REP will terminate when a JMP instruction leaves the set of instructions included in the REP zero overhead loop:
REP #@.FallThrough, count
shl D, S ‘instruction 0
cmp D, S WZ
if_Z jmp #out ‘when taken, cancels REP entirely
add D, S
.FallThrough JMP #elsewhere ‘executed when REP completes normally
[…]
out nop
add D, S
[…]
count long 5
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