Intel 80286, 80287 manual Appendix a System Initialization

APPENDIX A

80286 SYSTEM INITIALIZATION

flltl.('Swltch the 80286 from Re.1 Addr •• s Mode 10 Prolecled Mode')

nom. 'witch 80286~mode.

publiC ldl_de.c,gdt_d •• c

Switch the 80286 from real addr ••• mod. Into protect.d mode.

Th. lnilial EPROM GOT, lOT, TSS, .nd LOT (If any) con.tructed by BL0286 will be copied from EPROM In[o RAM. The RAM area •• ~e ~efln.d by data

••gm.nt. alloc.ted a. fixed entrle. In the GOT. The CPU r . gl . ter . for Ihe GOT, lOT, TSS, and LOT will be .el to point at the RAM-ba.ed

,"gm.nt •• Th. ba,e field. In the RAM-b •• ed GOT will 01.0 be updat.d to pOint at Ihe RAM-ba.ed segmenls.

Thl.code, is u.ed by adding It to the. Il . tof objeci module. glv.n to BL0286. BL0286 must Ih.n be told 10 place the •• gment

Inlt_code al addre •• FFFE10H. Execution of the mod. switch code begins after RESET. This happ.n. bec.u.e the mode switch cod. will .• torl at phy.lcal .ddre.s FFFFFOH, which Is the power up .dd ••••• This code th.n

. et . up RAM caples of Ihe EPROM-based .egmenls before jumping to the Initial la.k placed at • fixed GOT .ntry. Afler the Jump, the CPU

• x e c ut e. I n the s tate of t h • fir . t to. k d e fin. d by BL0 2 8 6 •

Thl. code will not us. any of Ih. EPROM-bas.d tables dlr.ctly. Such us. would r.sull In the 80286 writing Into EPROM to •• 1

the A bit. Any us. of a GOT or TSS will always be In the RAM copy. The limit and . I,e of the EPROM-based GOT and lOT mu.t b. stored at

the public .ymbols Idt_d •• c and gdl_d.sc. The location commands of BL0286 provld • . thl. function

Intorrupts are disabled during Ihls mode .wltchlng cod •• Full error ch.cklng I. m.de of the EPROM-ba •• d GOT, lOT, TSS, and LOT to a •• ur. th.y or. valid b.for. copying th.m to RAM. If any of the RAM-ba.ed alia •• egments are .m.ller Ihan the EPRDM .egm.nts th.y or. to hold, halt or .huldown will occur. In g.n.ral, any "exc.ptlon or Hi'll will cau.e shutdown to occur until the first ta.k Is Invok.d.

If the RAM s.gment I. larger Ihan the EPROM segm.nt, the RAM •• gm.nt will be expanded with ,era •• If th. Initial TSS specifies an LOT,

th. LOT will also b. copied Into ldt_alla. with zero fill If needed.

The EPRDM-ba •• d or RAM-ba.ed GOT, lOT, TSS, and LOT .egment. may b. locat.d anywhere In phy.lcal memory.

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