These illustrate an approach to initializing an address space suitable
for running a C program that uses newlib.
*** freestanding example apps:
stand00:
long-form %pcrel() assembler macros
stand01:
use pseudo-ops to implement crt0.S in an understandable manner
stand02:
- add some variables to the data and bss regions
- now there is somehting to zero-out
- note that the text region now is zero-padded in order to be able to initialize the page-aligned data region
- see the bss region get zeroed out this time
stand03:
call main --> main() { return 0x8675309 }
- the "call main" pseudo-operation is same as:
cm1: auipc x1,%pcrel_hi(main)
jalr x1,%pcrel_lo(callmain)(cm1)
- how does 'call' know to use x1 as the return address register??????
Because the ABI says so!
stand04:
call main() and set the value of a variable in the BSS
stand05:
call main() and set the values from variables in the DATA and BSS regions.
stand06:
Create a char array in main() that is not used. Even with an initializer it is optimized away!
stand07: (error)
Create char array in main() and then use it.
This examle fails due to missing memset()!!
stand08:
Add -lgcc and -lc libraries.
stand09: (error)
Add a call to malloc() that fails because there is no sbrk()
stand10: (note that the text region is too big to hardcode the data to start at 0x1000)
Add stubs for the newlib's stdlib dependancies in stub_stdlib.c
Demonstrate that malloc() is working properly.
Conclusion:
We can now execute single-threaded C programs that can use functions from the
standard library. There are no provisions, however, for things like I/O.
*** Related issues:
- use ABI names since x-this and x-that make thinking about arg-1 and arg-2 of a
C func calls easier. Show reg table from RVALP here.
- auipc + addi = how we put the address of some label into a register.
This method is optimized to eliminate accessing memory (where we could just store a pointer).
There is an 'la' pseudo-instruction:
la a1,target
... that tells the assembler to do this:
la1: auipc a1,%pcrel_hi(target)
addi a1,a1,%pcrel_lo(la1)
- pcrel_hi(label) represents an immediate-value suitable for auipc based on the label.
This immediate will be the distance from the PC to the label!!
- pcrel_lo(hi_label) is used to calc the right imm value for an addi insn matched to an auipc!
- lui + addi = how we put an absolute value into a register
There is an 'li' pseudo-instruction that can load any value into a register:
li a1,0x12345678
The li pseudo-operation on an RV32I will expand into this:
lui a1,%hi(0x12345678)
addi a1,a1,%lo(0x12345678)
On RV32 the assembler will only need to use either an addi or a lui and addi.
Note that 'li' works on RV64 and RV128 machines too. To accomplish the loading
the assembler will use a series of instructions like lui, addi, and shift.
