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Hello, I am using the SHARC 21065 processor. At the end of some of the code I have inherited, the following code exists: _exit: i12=dm(m7,i6); jump (m14,i12) (db); rframe; nop; .ENDSEG; Can anyone tell me what this is for? |
SHARC 21065
Started by ●February 28, 2005
Reply by ●March 1, 20052005-03-01
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On Mon, 28 Feb 2005, wadsworth_bob wrote: > I am using the SHARC 21065 processor. At the end of some of the code > I have inherited, the following code exists: > > _exit: > i12=dm(m7,i6); > jump (m14,i12) (db); > rframe; > nop; > .ENDSEG; > > Can anyone tell me what this is for? The manual says the following: CJUMP |function | (PC, <reladdr24>) | (DB); RFRAME; Function: The CJUMP instruction is generated by the C compiler for function calls, and is not intended for use in assembly language programs. CJUMP combines a direct or PC-relative jump with register transfer operations that save the frame and stack pointers. The RFRAME instruction reversed the register transfers to restore the frame and stack pointers. ... instruction what it does CJUMP function(DB) JUMP function (DB), R2=I6, I6=I7; RFRAME; I7 = I6, I6 = DM(0,I6) So it looks like you do a look up into a table, then jump to some offset from that address, and fix the registers i6 and i7 along the way. It's from a C compiler, so you may want to find the original source code to figure out what is really going on. Patience, persistence, truth, Dr. mike |
Reply by ●March 1, 20052005-03-01
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Mike Rosing wrote: >On Mon, 28 Feb 2005, wadsworth_bob wrote: > >>I am using the SHARC 21065 processor. At the end of some of the code >>I have inherited, the following code exists: >> >>_exit: >> i12=dm(m7,i6); >> jump (m14,i12) (db); >> rframe; >> nop; >>.ENDSEG; >> >>Can anyone tell me what this is for? >> >> > >The manual says the following: > >CJUMP |function | (PC, <reladdr24>) | (DB); >RFRAME; > >Function: >The CJUMP instruction is generated by the C compiler for function calls, >and is not intended for use in assembly language programs. CJUMP combines >a direct or PC-relative jump with register transfer operations that save >the frame and stack pointers. The RFRAME instruction reversed the >register transfers to restore the frame and stack pointers. > >... > >instruction what it does > >CJUMP function(DB) JUMP function (DB), R2=I6, I6=I7; > >RFRAME; I7 = I6, I6 = DM(0,I6) > >So it looks like you do a look up into a table, then jump to some offset >from that address, and fix the registers i6 and i7 along the way. It's >from a C compiler, so you may want to find the original source code to >figure out what is really going on. > >Patience, persistence, truth, >Dr. mike > > This is the "exit" macro used to maintain the run-time stack on return from a function call. I7 is the stack pointer and I6 is the frame pointer. The frame pointer points to the bottom of the current functions stack and the stack pointer points to the top where the free space is. The bottom of the current functions stack always contains the previous functions frame pointer. In the first instruction i6 contains the previous functions frame pointer and m7 contains the offset required to access the return address from the function, should be -1. The second instruction jumps to the return address + 1, as the return address is pointing to the second instruction of the delayed branch of the original function call, so m14 should contain a modifier of 1. The RFRAME instruction makes the stack pointer equal to the frame pointer. This releases the memory used for the current functions stack. As mentioned I6 points to the previous functions frame pointer. Thus the second part of the RFRAME instruction modifies the frame pointer. So the stack and frame pointers are now set up for the original calling function. Send instant messages to your online friends http://uk.messenger.yahoo.com |
Reply by ●March 1, 20052005-03-01
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--On Monday, February 28, 2005 8:02 PM -0800 Mike Rosing <> wrote: >> _exit: >> i12=dm(m7,i6); >> jump (m14,i12) (db); >> rframe; >> nop; >> .ENDSEG; > > CJUMP function(DB) JUMP function (DB), R2=I6, I6=I7; > > RFRAME; I7 = I6, I6 = DM(0,I6) It's been awhile and this is from memory: I7 is used as the C stack pointer. I6 is the frame pointer (which is used as a base pointer to access local variables and procedure arguments on the stack. CJUMP copies the frame pointer to R2 (a temporary) and another instruction will push it onto the stack, typically in the delay slots after the CJUMP. The code in _exit is a typical C return sequence. I12 is a temporary that can be used to address PM space. The return address is fetched using the frame pointer, the frame pointer and stack pointer are restored (using rframe), and the code jumps back to the caller. Take a look at the C manual to see the fixed values it puts in the M registers. I'm guessing that M7 is +1 and M14 is 0. I think the saved PC is stored just above (+1) the frame pointer (hence "(m7,i6)") and its value is the address of the next instruction in the calling code to execute (hence "(m14,i12)"). |
Reply by ●March 1, 20052005-03-01
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--On Monday, February 28, 2005 8:02 PM -0800 Mike Rosing <> wrote: >> _exit: >> i12=dm(m7,i6); >> jump (m14,i12) (db); >> rframe; >> nop; >> .ENDSEG; > > CJUMP function(DB) JUMP function (DB), R2=I6, I6=I7; > > RFRAME; I7 = I6, I6 = DM(0,I6) It's been awhile and this is from memory: I7 is used as the C stack pointer. I6 is the frame pointer (which is used as a base pointer to access local variables and procedure arguments on the stack. CJUMP copies the frame pointer to R2 (a temporary) and another instruction will push it onto the stack, typically in the delay slots after the CJUMP. The code in _exit is a typical C return sequence. I12 is a temporary that can be used to address PM space. The return address is fetched using the frame pointer, the frame pointer and stack pointer are restored (using rframe), and the code jumps back to the caller. Take a look at the C manual to see the fixed values it puts in the M registers. I'm guessing that M7 is +1 and M14 is 0. I think the saved PC is stored just above (+1) the frame pointer (hence "(m7,i6)") and its value is the address of the next instruction in the calling code to execute (hence "(m14,i12)"). |
