CALLA #\A Syntax Qestion
Bob Drury
Posts: 236
in Propeller 2
CALLA #\A Call to A by writing {C, Z, 10'b0, PC[19:0]} to hub long at PTRA++. If R = 1 then
PC += A, else PC = A. "\" forces R = 0.(404)
** What does PTRA++ mean? The following CALLA #\A example seems to work. Any Comments would be apreciated. **
CON {Processor Timing}
_clkfreq = 200_000_000 'processor clock speed
VAR
Byte cogStarted_COGEXEC_NEW 'cog ID started is returned or -1 if not started
OBJ
PUB main()
cogStarted_COGEXEC_NEW := COGINIT(COGEXEC_NEW,@CallProgram,PTRA_Stack)
repeat 'keep cog 0 running
DAT ORGH
PTRA_Stack long 10 'need 2 more characters than the stack length required
DAT ORG 0 'COGINIT(COGEXEC_NEW,@CallProgram,PTRA_Stack)
CallProgram
CALLA #Subroutine 'Compiler takes Subroutine-Call Program and generates offset
debug("Returned from Subroutine")
debug(udec(POPANum01),udec(POPANum02),udec(POPANum03))
debug(udec(POPANum04),udec(POPANum05),udec(POPANum06),udec(POPANum07))
debug(udec("Program Completed"))
Loop1 NOP
JMP #LooP1
Subroutine NOP
debug("Now In Subroutine")
WAITX clkTicks
debug("PUSH Num01 to Num07")
PUSHA Num01
PUSHA Num02
PUSHA Num03
PUSHA Num04
PUSHA Num05
PUSHA Num06
PUSHA Num07
WAITX clkTicks
debug("POP POPNum07 to POPNum01")
POPA POPANum07
POPA POPANum06
POPA POPANum05
POPA POPANum04
POPA POPANum03
POPA POPANum02
POPA POPANum01
RETA
Loop2 NOP
JMP #Loop2
clkTicks long $000A0_0000
Num01 long 1
Num02 long 2
Num03 long 3
Num04 long 4
Num05 long 5
Num06 long 6
Num07 long 7
POPANum01 long 0
POPANum02 long 0
POPANum03 long 0
POPANum04 long 0
POPANum05 long 0
POPANum06 long 0
POPANum07 long 0
Comments
PTRA register gets used as a "Stack Pointer", often called the SP register in other CPUs. CALLA places its return address on a stack in hubRAM. As opposed to CALL, which uses the hardware stack. PTRA contains the address of where in hubRAM to store this return address, ie: The stack.
Trivia: PTRA and PTRB are real registers too - most of cogRAM is made from SRAM cells. Therefore, there is also shadow-RAM behind PTRA and PTRB.
PTRA++ means the contents of PTRA which is incremented after use.
So if PTRA contains 1234 when the CALLA is executed, then {C, Z, 10'b0, PC[19:0]} is written to hubram at 1234 and then PTRA is incremented.
As a long is being written, PTRA++ in this case means add 4 to PTRA after the write.
Generally-speaking:
PTRx++ adds 4/2/1 after long/word/byte operation
PTRx-- subtracts 4/2/1 after long/word/byte op
++PTRx adds 4/2/1 before long/word/byte op
--PTRx subtracts 4/2/1 before long/word/byte op