__sbit is for SFRs, for regular data __bit should be used.
Otherwise the variables is not correctly declared in the BSEG
section, leading to bit temporaries being allocated in the same
location.
When this causes the tx_buf_empty to be overwritten to 0, then
the next write_char will hang forever.
The presence of the main function is a hardcoded trigger in sdcc that causes
the interrupt vectors and atomic rollback helpers to be generated as part of
compilation of the file containing the main function.
This makes sure the runtime library is correctly initialized and reduces
crtstart.asm to contain only the banked call helpers.
To ensure that the __interrupt_vect in area HOME is located at the beginning
of the firmware, the linker flags are updated to set the HOME start to
0x00000 and the linker object order is modified to have rtlplayground.rel
first.
Generated code in rtlplayground.asm:
```
;--------------------------------------------------------
; interrupt vector
;--------------------------------------------------------
.area HOME (CODE)
__interrupt_vect:
ljmp __sdcc_gsinit_startup
ljmp _isr_ext0
.ds 5
ljmp _isr_timer0
.ds 5
ljmp _isr_ext1
.ds 5
reti
.ds 7
ljmp _isr_serial
.ds 5
ljmp _isr_timer2
.ds 5
reti
.ds 7
reti
.ds 7
ljmp _isr_ext2
.ds 5
ljmp _isr_ext3
; restartable atomic support routines
.ds 2
sdcc_atomic_exchange_rollback_start::
nop
nop
sdcc_atomic_exchange_pdata_impl:
movx a, @r0
mov r3, a
mov a, r2
movx @r0, a
sjmp sdcc_atomic_exchange_exit
nop
nop
sdcc_atomic_exchange_xdata_impl:
movx a, @dptr
mov r3, a
mov a, r2
movx @dptr, a
sjmp sdcc_atomic_exchange_exit
sdcc_atomic_compare_exchange_idata_impl:
mov a, @r0
cjne a, ar2, .+#5
mov a, r3
mov @r0, a
ret
nop
sdcc_atomic_compare_exchange_pdata_impl:
movx a, @r0
cjne a, ar2, .+#5
mov a, r3
movx @r0, a
ret
nop
sdcc_atomic_compare_exchange_xdata_impl:
movx a, @dptr
cjne a, ar2, .+#5
mov a, r3
movx @dptr, a
ret
sdcc_atomic_exchange_rollback_end::
sdcc_atomic_exchange_gptr_impl::
jnb b.6, sdcc_atomic_exchange_xdata_impl
mov r0, dpl
jb b.5, sdcc_atomic_exchange_pdata_impl
sdcc_atomic_exchange_idata_impl:
mov a, r2
xch a, @r0
mov dpl, a
ret
sdcc_atomic_exchange_exit:
mov dpl, r3
ret
sdcc_atomic_compare_exchange_gptr_impl::
jnb b.6, sdcc_atomic_compare_exchange_xdata_impl
mov r0, dpl
jb b.5, sdcc_atomic_compare_exchange_pdata_impl
sjmp sdcc_atomic_compare_exchange_idata_impl
```
This saves 508 bytes in the HOME area by:
- Getting rid of the expensive hex array indexing
- Having print_short and print_long delegate to print_byte
This is at the expense of increasing the stack depth a tiny bit.
Stack usage isn't an issue so far since I see ~96 bytes of
untouched stack memory.
This adds the MPAGE (sdcc _XPAGE) SFR, which is used by sdcc in `--xstack`
mode to provide the upper byte of the address for `MOVX A, @Ri` and
`MOVX @Ri, A` instructions.
Classic 8051 uses the P2 SFR value for this, but the RTL IP block does not
seem to be using the "Port 2 module".
From the databook:
```
To replace the function of the Port 2 latch in designs that do not use a
Port 2 module, the DW8051 provides an additional special function register,
MPAGE, at SFR address 92h. During MOVX A, @Ri and MOVX @Ri, A instructions,
the DW8051 places the contents of the MPAGE register on the upper 8 address
bits (mem_addr[15:8]). This provides the paging function that is normally
provided by the Port 2 latch. The MPAGE register has no function when a
Port 2 module is used to connect external RAM.
```
Tobias Diedrich