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Explain the booterr code a bit

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Nero 2020-04-03 19:43:10 +00:00
parent 3da473a00c
commit 6eca218f8e
1 changed files with 12 additions and 3 deletions
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15
inc/booterr.asm
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@ -1,18 +1,27 @@
errcll: pop dx errcll: ; put return address into dx
pop dx
errjmp: call dmpdx errjmp: call dmpdx
hlt: hlt hlt: hlt
jmp hlt jmp hlt
dmpdx: xor bx, bx dmpdx: ; setup bx and ah for int 10h call
xor bx, bx
mov ah, 0x0e mov ah, 0x0e
mov cl, 4 mov cl, 4
; this double-call is essentially a 4 times repeating loop
call dmpdx1 call dmpdx1
dmpdx1: call dmpdx2 dmpdx1: call dmpdx2
dmpdx2: mov al, dh dmpdx2: ; grab highest nibble from dx
mov al, dh
; remove highest nibble from dx
shl dx, cl shl dx, cl
; shift away second-highest nibble that we accidentally copied
shr al, cl shr al, cl
; map 0-9 to ascii codes for '0' to '9'
add al, 0x30 add al, 0x30
; if result is larger than '9', ...
cmp al, 0x3a cmp al, 0x3a
jl dmpdx3 jl dmpdx3
; ... add 7 so we continue at 'A'
add al, 7 add al, 7
dmpdx3: int 0x10 dmpdx3: int 0x10
ret ret