/joh'liks/ n.,adj. 386BSD
We stepwise proved out the running environment of program tools, program loading and execution, trap handling, stack, and support of high level language. Our project moved from fragile to substantive.
|
Listing 1: Simplest protected mode program providing output
# File "hi.s": .text start: # put "hi" mid screen on display movl $0x0e690e48, 0x0b8800 hlt While this may seem trite to some, this program illustrates the pathetic level at which untested software tools begin. After eliminating a handful of nuisance bugs, this simple program did work, and it proved valuable because it was able to smoke out bugs quickly. DOS "MASM" vs UNIX "as": Assembler Syntax Difference
A side note to those who may have noticed that our assembly code format seems to have changed since the previous article when we used Microsoft's MASM: For those unaware, UNIX 386 assemblers prefer the operands in the opposite order, partly because early UNIX systems appeared on PDP-11s, which preferred this ordering style. Thus, on MS-DOS with MASM:mov eax,edx ;move 32 bit edx into eax registercorresponds to the UNIX assembler format: movl %edx,%eax #move 32 bit edx into eax registerIn other words, it is (destination, source) instead of (source, destination). This is yet another stunning "improvement" in the field of computer languages, destined to be appreciated by those simultaneously debugging a MASM-coded bootstrap loader and code generated by the GAS UNIX assembler! Listing 2: Standalone "Hello World" program
# File "hello.s": # Minimal test of GNU GAS assembler, # handles CGA & strings. .text start: movl $0xA0000,%esp pushl $str call _puts pop %eax hlt str: .asciz "\n\rHello world from GAS\r\n" _puts: push %ebx movl 8(%esp),%ebx 1: cmpb $0,(%ebx) # until we see a null je 2f movzbl (%ebx),%eax pushl %eax call _putchar # put out characters popl %eax incl %ebx jmp 1b 2: popl %ebx ret crtat: .long 0xb8000 # address of CGA video RAM row: .long 0 _putchar: movzbl 4(%esp),%eax push %ebx push %ecx movl crtat,%ebx # continous output off screen edge & bot cmpl $0xb8000+80*25*2,%ebx jl 1f movl $0,row movl $0xb8000+80*(25-1)*2,%ebx 1: cmpb $0xd,%al # cr jne 1f movl $80,%ecx # clear rest of line subl row,%ecx movl %ebx,%edi movw $0xfff,%ax cld rep stosw subl row,%ebx subl row,%ebx movl $0,row jmp 9f 1: cmpb $0xa,%al # nl jne 2f cmpl $0xb8000+80*(25-1)*2,%ebx # scroll? jl 1f movl $0xb8000,%edi # scroll page movl $0xb8000+80*2,%esi movl $80*(25-1),%ecx cld rep movsw movl $80,%ecx # clear new bottom line movl $0xb8000+80*(25-1)*2,%edi movw $0,%ax rep stosw sub $80*2,%ebx # position cursor before lf 1: add $80*2,%ebx jmp 9f 2: orw $0x0e00,%ax # attribute movw %ax,(%ebx) addl $2,%ebx incl row 9: movl %ebx,crtat pop %ecx pop %ebx ret Listing 3: Standalone runtime and C "Hello World"
# [Excerpted from file: srt.s]
...
entry: .globl entry
jmp 1f
.space 0x500 # skip over BIOS data area
1: cli # no interrupts yet
movl $0xA0000,%esp
movl %esp,%edx
movl $_edata,%eax
subl %eax,%edx # clear stack and heap store
pushl %edx
pushl %eax
call _bzero
popl %eax
popl %eax
call _main
...
/* File: hello.c */
main() { printf("Hello, world!\n"); }
We can't stress enough the need to "prove out" the running environment prior to handling larger efforts. Its much easier to gradually bring up an absolute programming environment this way rather than attempt to work through compound errors - especially on a machine that will hand you a processor restart fault, dump you back in the BIOS without a hope in hell of knowing where it came from.
|
|