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kernel
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fix bug#123, detect NEC V20/V30 as 186 instead of only 8086/8088, based on ecm-pushbx ldebug 

see https://hg.pushbx.org/ecm/ldebug/file/7f3440d5824d/source/init.asm#l3071

(cherry picked from commit 5fdcc1d214b821fde33a5bd92ea7d7c0134a2056)
This commit is contained in:
Kenneth J Davis 2024-02-03 22:19:02 -05:00
parent 431be02b0a
commit 07155e6184
1 changed files with 16 additions and 0 deletions
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16
kernel/cpu.asm
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@ -48,6 +48,22 @@ CPU 386
and ax, 0f000h
cmp ax, 0f000h
jnz is286 ; no the 4 msb stuck set to 1, so is a 808x or 8018x
; NEC V20/V30 support 186 instructions but
; do not mask the shift count like a 186.
; based on https://hg.pushbx.org/ecm/ldebug/file/7f3440d5824d/source/init.asm#l3071
; which is based on http://www.textfiles.com/hamradio/v20_bug.txt
mov ax, sp ; we use stack to do test
mov cx, 1 ; after pop still 1 if 8088/8086
push cx
dec cx ; after pop still 0 if NEC V20/V30
; next instructions may lock system if breakpoint or trace flag set
db 8Fh, 0C1h; pop r/m16 with operand cx on 808x, nop on NEC V20/V30
mov sp, ax ; reset stack to known good state (pre push, optional pop)
xor cx, cx ; cx is 1 if NEC, 0 if 808x
jz is808x
mov bx, cx
jmp short cleanup
is808x:
mov ax,1 ; determine if 8086 or 186
mov cl,64 ; try to shift further than size of ax
shr ax,cl