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----- Original Message ----- From: jrem123 <> To: <> Sent: Tuesday, February 03, 2004 4:31 PM Subject: [piclist] more PC, PCL, and PCLATH confusion > Okay, so I can write into the PCL, and I can write into the PCLATH, > and dependent upon the execution either an 11 bit or a 13 bit value > goes into the PC (which is PCH and PCL). Don't think of it as "either / or" just think of it as always the same but with some of the bits not having any effect. > > 11 bits is 7FF, 13 bits is 1FFF (I'm working with a 16F628). So it > appears the 1FFF is the 2k flash program size, $800 is 2K, $7FF is 2K-1 > so there are 4 banks > of 7FF each bank, so an 11 bit wide PC stays in the same bank, a 13 > bit PC jumps banks? Its always 13 bits but with a 628 all 4 banks map to bank 0 > > Now, assuming I am somewhat close on the above, why are the banks > 7F, FF, 17F, and 1FF? I would think they would be 7FF, FFF, 17FF, > and 1FFF? You are confusing RAM banks and code banks. > > What am I missing? Yes. The PIC has two seperate memories for variable and program storage. The variable memory (RAM and special function registers) start at address 0 and end at address $1FF. The program memory starts at a DIFFERENT address 0 and ends at address $1FFF. The instructions in a program that access the variable memory can only be used to access the variable memory. The instructions that access program memory can only access program memory. RP0 and RP1 in the STATUS register select the current variable memory bank, Bits 11 and 12 of the PC select the current program memory bank. Bits 11 and 12 of the PC are loaded from PCLATH whenever you execute a GOTO or call instruction or when you write to PCL. Within a variable memory accessing instruction (e..g. ADDWF) you have the address field which is made up of 7 bits, this holds a number between 0 and $7F. RP0 forms the eighth bit if this address and RP1 forms the nineth bit of this address. When the variable memory is accessed all 9 bits of this address are used. Within a program memory accessing instruction (e.g. GOTO or CALL) you have the address field which is made up of 11 bits. This holds a number between 0 and $7FF. Bit 11 of PCLATH forms bit 11 of this address and bit 12 of PCLATH forms bit 12 of this address. So the 11 bits in the instruction now look like 13 bits. When the instruction is executed all 13 bits are loaded into the PC. FSR can only be used to reference the variable memory. It is 8 bits wide so can only address $00 to $FF. To read $100 to $1FF you need to use IRP in the STATUS register. Clear IRP to 0 and FSR refers to address $00 to $FF in the variable menory, set IRP to 1 and FSR refers to address $100 to $1FF in the variable menory. IRP behaves like a nineth bit for FSR. FYI the xcasm assembler does automatic RAM and code bank management for you. It inserts RAM and code bank select instructions into the executable. The xcsim simulator traps variable accesses in the wrong RAM bank and code accesses to the wrong code bank. Regards Sergio Masci http://www.xcprod.com/titan/XCSB - optimising structured PIC BASIC compiler |