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Пишет nancygold (![[info]](http://lj.rossia.org/img/userinfo.gif){kind=small} nancygold)@ 2023-09-25 17:10:00 |
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| Настроение: |  amused |
| Entry tags: | computing |
RISC OS "Hello World"
Generally one can just write raw code to AIF (file type is external anyway).
Yet C/C++ ABI requires the following header to be present.
C library's _start looks into it to determine load parameters.
The code also have to be re-entrant, so that OS can cache single AIF among several processes.
So after unpacking and relocation, jumps to these routines get replaced by NOPs.
AIF is considered obsolete, and they are trying to force everyone to use Linux's ELF.
But modern RISC OS does support it through half-emulation.
In fact, you can still run 90ies build of Heroes of Might & Magic II on your raspberry pi.
I like AIF more than the more formal ELF. It offers more freedom. Like a blank list of paper.
The classic ARM assembler uses different terminology,
Segments/Sections = areas
Trampolines = veneers
Generally, ARM is not purely RISC, but half-way RISC.
Especially its modern version, which feature variable sized byte code.
Below is an example of hello world console app.
Assembler included initialization code because it is part of well formed C ABI header, but you can just remove it.
Still I commented it. Although it is not a good code, since .bss is aligned to 16 bytes, so we can do STM instead of STR.
STM and LDM is the ARM's way to do memcpy/memset, push/pop stacks and load/save C structs.
There are also SIMD instructions working in parallel with STM/LDM over normal registers.
Obviously STM/LDM is a CISC opcode, so yeah, ARM is not fanatically RISCy.
00008000 00 00 a0 e1 mov r0,r0 ; optional decompression code
00008004 00 00 a0 e1 mov r0,r0 ; optional relocation code
00008008 00 00 a0 e1 mov r0,r0 ; optional init code
0000800c 1b 00 00 eb bl entry void entry(void)
00008010 11 00 00 ef swi 0x11
ro_sz
00008014 ac 00 00 00 ddw ACh
rw_sz
00008018 00 00 00 00 ddw 0h
debug_sz
0000801c 00 00 00 00 ddw 0h
zero_sz
00008020 00 00 00 00 ddw 0h
debug_type
00008024 00 00 00 00 ddw 0h
base
00008028 00 80 00 00 ddw 8000h
workspace
0000802c 00 00 00 00 ddw 0h
mode ; CPU architecture
00008030 1a 00 00 00 ddw 1Ah
data_base
00008034 00 00 00 00 ddw 0h
reserved1
00008038 00 00 00 00 ddw 0h
reserved2
0000803c 00 00 00 00 ddw 0h
bl_debug_init
00008040 00 00 a0 e1 mov r0,r0
**************************************************************
* FUNCTION *
**************************************************************
void zero_init(void) ;this one clears .bss segment
void
zero_init ;we BL here from 8008, when it is non NOP
00008044 0f c0 4e e0 sub r12,lr,pc ;r12 = 0x800c - 0x804c = -0x40
00008048 0c c0 8f e0 add r12,pc,r12 ;r12 = 0x8050 + -0x40 = 0x8010
0000804c 0f 00 9c e9 ldmib r12,{r0,r1,r2,r3} ;read 4 values from [0x8010 + 4 = 8014]
;b means add 4 to r12, before reading
;r0=ro_sz, r1=rw_sz, r2=debug_sz, r3=zero_sz
00008050 10 c0 4c e2 sub r12,r12,#0x10 ;0x8020 - 0x10 = 0x8010
00008054 30 20 9c e5 ldr r2,[r12,#0x30] ;r2 = word from bl_debug_init
00008058 01 0c 12 e3 tst r2,#0x100 ;test debug bit
0000805c 34 c0 9c 15 ldrne r12,[r12,#0x34] ;init destination based on on debug opcode
00008060 00 c0 8c 00 addeq r12,r12,r0 ;r12 + ro_sz
00008064 01 c0 8c e0 add r12,r12,r1 ;r12 + rw_sz
00008068 00 00 a0 e3 mov r0,#0x0 ;value we will be clearing memory with
0000806c 00 00 53 e3 cmp r3,#0x0 ;is .bss empty?
LAB_00008070 XREF[1]: 0000807c(j)
00008070 0e f0 a0 d1 movle pc,lr ;if (!count) return
00008074 04 00 8c e4 str r0,[r12],#0x4 ;*bss_ptr++ = 0
00008078 04 30 53 e2 subs r3,r3,#0x4 ;count -= 4
0000807c fb ff ff ea b LAB_00008070
**************************************************************
* FUNCTION *
**************************************************************
void entry(void)
void
entry XREF[1]: 0000800c(c)
00008080 01 00 00 ef swi 0x1 ; OS_WriteS
00008084 48 65 6c ds "Hello, world!"
6c 6f 2c
20 77 6f
00008094 03 00 00 ef swi 0x3 ; OS_NewLine
00008098 00 00 a0 e3 mov r0,#0x0 ; pointer to error buffer
0000809c 04 10 9f e5 ldr r1,[abex] ; ABEX = set return code
000080a0 00 20 a0 e3 mov r2,#0x0 ; return code
000080a4 11 00 00 ef swi 0x11 ; OS_Exit
abex XREF[1]: 0000809c(R)
000080a8 41 char 'A'
000080a9 42 char 'B'
000080aa 45 char 'E'
000080ab 58 char 'X'
