rdos/kernel/printf.asm

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section .bss
printr:
.ax: resw 1
.cx: resw 1
.dx: resw 1
.bx: resw 1
section .text
; Stack contents
; ret-addr arg1 arg2 arg3 ...
printf: mov [printr.ax], ax
mov [printr.cx], cx
mov [printr.dx], dx
mov [printr.bx], bx
pop bx
.loop: mov al, [cs:bx]
inc bx
cmp al, 0
je .end
cmp al, 2
je .word
call pputc
jmp .loop
.end: push bx
mov ax, [printr.ax]
mov cx, [printr.cx]
mov dx, [printr.dx]
mov bx, [printr.bx]
ret
.word: pop dx
call pdx
jmp printf.loop
pdx: ; this double-call is essentially a 4 times repeating loop
call .l1
.l1: call .l2
.l2: ; set up cl for bit shifts
mov cl, 4
; grab highest nibble from dx
mov al, dh
; remove highest nibble from dx
shl dx, cl
; shift away second-highest nibble that we accidentally copied
shr al, cl
; map 0-9 to ascii codes for '0' to '9'
add al, 0x30
; if result is larger than '9', ...
cmp al, 0x3a
jl pputc
; ... add 7 so we continue at 'A'
add al, 7
pputc: push bx
mov ah, 0x0e
xor bx, bx
int 0x10
pop bx
ret