The ROM(small/compact/large) directive, selected by the "Code Rom Size" option in the uVision Target dialog, tells the compiler to generate ACALL / AJMP instructions inside functions (compact) or for the whole program (small). There is also a "linker code packing" checkbox on the C51 optimization options. This optimization allows the linker to rearrange code to use the short instructions where possible, replacing LCALL / LJMP with the Ax versions. Is the ROM directive thus redundant? Or is there some additional benefit to the code generated with the ROM(compact) memory model selection compared to ROM(large)? (I'm contemplating switching from the large model to small. I have most of my variables explicitly assigned to a memory space, and make heavy use of overlaid data for auto variables (the "stack"). This noticably improves code size over the default to xdata. However, it seems that parameters that spill out of registers, and compiler-generated temporaries will still appear in xdata, with all the painful access that implies. So I was hoping a switch to the small model would move these locations into internal RAM as well, and buy me a little more code space. Hopefully it doesn't blow my "stack" size altogether.)
Is the ROM directive thus redundant? Or is there some additional benefit to the code generated with the ROM(compact) memory model selection compared to ROM(large)? No. The ROM directive is not redundant. However, if you allows the LX51 linker to substitute shorter JMP and CALL instructions, its use it unimportant (since the linker moves things around to maximize AJMPs and ACALLs. ROM is useful in cases where you don't want this level of linker optimization or in cases where you don't have the LX51 linker. The real difference is that the ROM directive forces you to maintain the size of functions or modules while using the linker does not. Jon
"The real difference is that the ROM directive forces you to maintain the size of functions or modules" Does the compiler warn you if your functions/modules get too big?
I believe you get a segment too large error but I haven't tested it. Jon
Thanks for the clarifications.