;============================================================================= ; boot.asm - Protected mode example boot program. ; ; Copyright (C) 1992-93, Dennis Marer File created: 12/21/92 ; Last modified: 1/27/93 ; ; Description: ; This program sets up the system, the hardware, and the 386 for use in ; its mysterious and tricky Protected Mode. This initializes the GDT, the ; IDT, the kernel TSS, and an LDT for use by the kernel. Hardware which ; is initialized includes the 8259 interrupt controller (PIC), the 8253 ; timer (PIT), and the 8042 slave processor (SP) for A20 control. The ; final long jump into protected mode begins the execution of the kernel. ; ; Portability: ; Requires Turbo Assembler 1.0 (TASM) or better to be assembled. ; Dependent on the IBM PC 386 or better platform and its standard devices. ; ; Modifications: ; DPM, 1/27/93 Finished the first revision of this file. ;============================================================================= .386p include 386p.inc ; 386 Protected mode include ibmpc.inc ; IBM PC hardware include kernel.inc ; Kernel definitions ;============================ Boot stack segment ============================= BootStack segment para stack use32 'BootStack' dw 64 dup (?) BootStack ends ;============================= Boot data segment ============================= BootData segment use16 'BootData' ;----------------------------------------------------------------------------- ; Miscellaneous data, used only during initialization. ;----------------------------------------------------------------------------- SPFailure db 'Failure de-gating the A20 line.$' segBase dd 0 ; Linear segment base PIC1Mask db 0 ; Saved interrupt masks PIC2Mask db 0 GDTInfo dw GDTEnd-GDTStart-1 ; GDT limit GDTAddr dd 0 ; GDT linear start address IDTInfo dw IDTEnd-IDTStart-1 ; IDT limit IDTAddr dd 0 ; IDT linear start address ;----------------------------------------------------------------------------- ; List of interrupt handlers used to initialize the vectors. ;----------------------------------------------------------------------------- intrTable: dw IntrDivide ; 0 - Divide error dw IntrDebug ; 1 - Debug exception dw IntrNMI ; 2 - NMI dw IntrBreakpoint ; 3 - One byte interrupt dw IntrOverflow ; 4 - Interrupt on overflow dw IntrBounds ; 5 - Array bounds error dw IntrOpcode ; 6 - Invalid opcode dw IntrMathGone ; 7 - Math not available dw IntrDouble ; 8 - Double fault dw IntrMathOver ; 9 - Math segment overflow dw IntrTSS ; 10 - Invalid TSS dw IntrSegment ; 11 - Segment not present dw IntrStack ; 12 - Stack fault dw IntrGeneral ; 13 - General protection dw IntrPage ; 14 - Page fault dw Reserved ; 15 dw IntrMathErr ; 16 - Math error dw Reserved ; 17 dw Reserved ; 18 dw Reserved ; 19 dw Reserved ; 20 dw Reserved ; 21 dw Reserved ; 22 dw Reserved ; 23 dw Reserved ; 24 dw Reserved ; 25 dw Reserved ; 26 dw Reserved ; 27 dw Reserved ; 28 dw Reserved ; 29 dw Reserved ; 30 dw Reserved ; 31 dw KernelClock ; 32 - IRQ 0 dw Unused ; 33 - IRQ 1 dw Unused ; 34 - IRQ 2 dw Unused ; 35 - IRQ 3 dw Unused ; 36 - IRQ 4 dw Unused ; 37 - IRQ 5 dw Unused ; 38 - IRQ 6 dw Unused ; 39 - IRQ 7 dw Unused ; 40 - IRQ 8 dw Unused ; 41 - IRQ 9 dw Unused ; 42 - IRQ 10 dw Unused ; 43 - IRQ 11 dw Unused ; 44 - IRQ 12 dw Unused ; 45 - IRQ 13 dw Unused ; 46 - IRQ 14 dw Unused ; 47 - IRQ 15 intrTableSize equ ($ - intrTable) / 2 ; Size (number of entries) ;----------------------------------------------------------------------------- ; Global Descriptor Table (GDT) - Set up the GDT for use in Protected Mode, ; including a descriptor for the kernel code and data segments, the kernel ; TSS, the necessary stack fault handler TSS and double handler TSS, and ; a descriptor to access the screen memory. ;----------------------------------------------------------------------------- GDTStart seg_descr <0,0,0,0,0,0> ; NULL descriptor _access = AC_MAPPED+ACT_A386 ; PM TSS descriptor kernelDescr seg_descr _access = AC_MAPPED+AC_USER+AC_CODE+ACC_READ ; Kernel code segment descrKCS seg_descr <0FFFFh,0,0,_access,LIM_32BIT,0> _access = AC_MAPPED+AC_USER+ACD_WRITE ; Kernel data segment descrKDS seg_descr <0FFFFh,0,0,_access,0,0> _access = AC_MAPPED+ACT_A386 ; Stack fault handler descrStack seg_descr _access = AC_MAPPED+ACT_A386 ; Double fault handler descrDouble seg_descr _access = AC_MAPPED+ACT_LDT ; LDT descriptor descrLDT seg_descr _access = AC_MAPPED+AC_USER+ACD_WRITE ; Screen memory descrScreen seg_descr <0ffffh,0,0,_access,0,0> _access = AC_MAPPED+AC_USER+ACD_WRITE ; Memory over 1 meg! descr1Meg seg_descr <0ffffh,0,0,_access,0,0> GDTEnd equ $ ;----------------------------------------------------------------------------- ; Interrupt descriptor Table, which will be filled in later. ;----------------------------------------------------------------------------- IDTStart db (32+16)*8 dup (?) IDTEnd equ $ ;----------------------------------------------------------------------------- ; Empty LDT table, required by Protected Mode for the task. ;----------------------------------------------------------------------------- LDTStart seg_descr <0,0,0,0,0,0> ; NULL descriptor LDTLimit equ $ - LDTStart ;----------------------------------------------------------------------------- ; Task State Segments. ;----------------------------------------------------------------------------- kernelTSS tss386 <> TSSLimit equ $ - kernelTSS - 1 ; Limit of a TSS TSSStack tss386 <> ; Stack fault handler TSSDouble tss386 <> ; Double fault handler ;----------------------------------------------------------------------------- ; Define each of the selectors for the useful segments. ;----------------------------------------------------------------------------- selKernel equ kernelDescr - GDTStart selKCS equ descrKCS - GDTStart selKDS equ descrKDS - GDTStart selStack equ descrStack - GDTStart selDouble equ descrDouble - GDTStart selLDT equ descrLDT - GDTStart selScreen equ descrScreen - GDTStart sel1Meg equ descr1Meg - GDTStart BootData ends ;============================================================================= Boot segment para use16 'Boot' assume CS:Boot,DS:BootData ;----------------------------------------------------------------------------- ; LINEAR - Create a 32-bit linear address of the specified variable and ; store the result in EAX. This relies on the segment base in segBase. ; This is destructive to the EBX register. ;----------------------------------------------------------------------------- LINEAR macro variable mov EAX,dword ptr segBase mov EBX,offset variable add EAX,EBX endm ;----------------------------------------------------------------------------- ; DBASE - Macro to store the base address held in EAX into the specified ; descriptor. This is destructive to the EAX register. ;----------------------------------------------------------------------------- DBASE macro dest mov dest.base_0,AX ; Store the bits 0-15 shr EAX,16 mov dest.base_16,AL ; Store the bits 16-19 endm ;----------------------------------------------------------------------------- ; DiskOff - Force the floppy drive to shut its motor off by calling the ; interrupt 8 (IRQ 0) handler a good number of times. This also speeds ; up the system clock a strange amount. It's a hack, but it works. ;----------------------------------------------------------------------------- DiskOff proc near mov CX,1000 ; Loop 1000 times DiskOffLoop: int 8 ; Call IRQ 0 handler loop DiskOffLoop ret DiskOff endp ;----------------------------------------------------------------------------- ; SetupGDT - Setup the Global Descriptor Table (GDT) for use within the ; 386 Protected Mode, initializing each of the descriptors and the Task ; State Segments (TSS) found there. ;----------------------------------------------------------------------------- SetupGDT proc near LINEAR GDTStart ; Save the linear address of mov GDTAddr,EAX ; the GDT for later xor EAX,EAX ; Create a 32-bit linear mov AX,seg KernelStart ; address for the kernel shl EAX,4 ; code segment and place DBASE descrKCS ; this in its descriptor xor EAX,EAX ; Create a 32-bit linear mov AX,seg initStack ; address for the kernel shl EAX,4 ; data segment and place DBASE descrKDS ; this in its descriptor LINEAR kernelTSS ; Create the linear addresses DBASE kernelDescr ; for each of the TSS LINEAR TSSStack ; which reside in this DBASE descrStack ; program's data segment LINEAR TSSDouble DBASE descrDouble LINEAR LDTStart ; Also the LDT DBASE descrLDT mov EAX,0B8000h ; Screen fixed in memory DBASE descrScreen mov EAX,100000h ; Memory over 1 megabyte! DBASE descr1Meg lea DI,kernelTSS ; Set up the kernel TSS mov AX,selKDS mov [DI].t3SS0,AX mov [DI].t3SS1,AX mov [DI].t3SS2,AX mov [DI].t3DS,AX mov [DI].t3ES,AX mov [DI].t3FS,AX mov [DI].t3GS,AX mov [DI].t3SS,AX mov [DI].t3CS,selKCS mov [DI].t3ESP0,offset stack0 mov [DI].t3ESP1,offset stack1 mov [DI].t3ESP2,offset stack2 mov [DI].t3LDT,selLDT mov [DI].t3EFlags,000000h ; Interrupts off cld ; Set direction to forward lea SI,kernelTSS ; Duplicate the PM TSS into lea DI,TSSStack ; the stack fault TSS mov CX,TSSLimit+1 rep movsb mov TSSStack.t3EIP,offset IntrStack lea SI,kernelTSS ; Duplicate the PM TSS into lea DI,TSSDouble ; the double fault TSS mov CX,TSSLimit+1 rep movsb mov TSSDouble.t3EIP,offset IntrDouble ret SetupGDT endp ;----------------------------------------------------------------------------- ; SetupIDT - Setup the Interrupt Descriptor Table (IDT) for use within the ; 386 Protected Mode, initializing each of the descriptors with values ; from the existing interrupt vectors. ;----------------------------------------------------------------------------- SetupIDT proc near LINEAR IDTStart ; Save the linear address of mov IDTAddr,EAX ; the IDT for later mov CX,intrTableSize ; Set things up to loop lea BX,intrTable ; through the whole lea DI,IDTStart ; interrupt table FillIDT: mov [DI].selector,selKCS ; Selector value to use mov [DI].words,00h ; No words to be pushed mov [DI].access,08Eh ; Descriptor type (int gate) mov AX,[BX] ; Set the interrupt handler mov [DI].offset_0,AX mov [DI].offset_16,0 add DI,8 ; Move on to the next vector add BX,2 ; and the next table entry loop FillIDT lea DI,IDTStart+(8*8) ; Vector 8, double fault mov [DI].selector,selDouble ; Selector value to use mov [DI].words,00h ; No words to be pushed mov [DI].access,0E5h ; Task Gate with DPL=3 mov [DI].offset_0,0 ; (unused) mov [DI].offset_16,0 ; (unused) lea DI,IDTStart+(12*8) ; Vector 12, stack fault mov [DI].selector,selStack ; Selector value to use mov [DI].words,00h ; No words to be pushed mov [DI].access,0E5h ; Task Gate with DPL=3 mov [DI].offset_0,0 ; (unused) mov [DI].offset_16,0 ; (unused) ret SetupIDT endp ;----------------------------------------------------------------------------- ; EmptySP - Wait for the 8042 Slave Processor (SP) to empty its buffer. This ; is used exclusively by the SetupSP procedure. ; ; Returns AL=0 if the buffer is empty, timeout occurred otherwise. ;----------------------------------------------------------------------------- EmptySP proc near push CX ; Save used registers xor CX,CX ; CX=65536 for timeout value EmptySPLoop: in AL,SP_STATUS ; Read the status port, find and AL,SP_BUFFER_FULL ; out if the buffer is full loopnz EmptySPLoop ; Loop until buffer empty pop CX ; Restore used registers ret EmptySP endp ;----------------------------------------------------------------------------- ; SetupSP - Setup the 8042 Slave Processor (SP), most notably enabling its ; A20 output to allow access to memory above 1 megabyte. If this function ; fails, a message is displayed and the program terminates. ;----------------------------------------------------------------------------- SetupSP proc near call EmptySP ; Empty the 8042 SP buffer, jnz SetupSPExit ; exit on failure mov AL,SP_WRITE_OUTPUT ; Send command to write out SP_STATUS,AL ; output port, wait until call EmptySP ; the 8042 buffer is empty, jnz SetupSPExit ; exit on failure mov AL,SP_ENABLE_A20 ; Send command to enable the out SP_PORT_A,AL ; A20 line, wait until the call EmptySP ; 8042 buffer is empty, jnz SetupSPExit ; exit on failure ret SetupSPExit: sti ; Re-enable interrupts mov AH,009h ; Output failure message mov DX,offset SPFailure int 21h int 20h ; Terminate program SetupSP endp ;----------------------------------------------------------------------------- ; SetupPIC - Setup the dual 8259 Programmable Interrupt Controllers (PIC) ; to relocate the hardware interrupts past the 386 reserved vectors. ;----------------------------------------------------------------------------- PIC1_VECTOR equ 020h ; New PIC interrupt vectors PIC2_VECTOR equ 028h SetupPIC proc near in AL,PIC1_DATA ; Save the current mask mov PIC1Mask,AL ; state of both of the in AL,PIC2_DATA ; 8259 PICs mov PIC2Mask,AL mov AL,ICW1_INIT+ICW1_ICW4 ; Send the initialization out PIC1_CMND,AL ; sequence to both the DELAY ; 8259 PICs out PIC2_CMND,AL DELAY mov AL,PIC1_VECTOR ; Set the new interrupt out PIC1_DATA,AL ; vectors to avoid DELAY ; conflicts with the mov AL,PIC2_VECTOR ; 386 reserved vectors out PIC2_DATA,AL DELAY mov AL,4 ; PIC1 is the master, which out PIC1_DATA,AL ; has channel 2 cascaded DELAY mov AL,2 ; PIC2 is slave, with ID #2 out PIC2_DATA,AL DELAY mov AL,ICW4_8086 ; Put both PICs into 8086 out PIC1_DATA,AL ; mode, to use vectors DELAY out PIC2_DATA,AL DELAY mov AL,PIC1Mask ; Restore the interrupt masks out PIC1_DATA,AL ; to their original states DELAY mov AL,PIC2Mask out PIC2_DATA,AL DELAY ret SetupPIC endp ;----------------------------------------------------------------------------- ; SetupPIT - Setup the dual 8253 Programmable Interval Timer (PIT) to speed ; up the system clock (timer 0) to roughly 100 ticks/second. The other ; two timers are still used for DMA (timer 1) and speaker (timer 2). ;----------------------------------------------------------------------------- SetupPIT proc near mov AL,PIT_SC0+PIT_LOAD+PIT_SQUARE out PIT_CMND,AL mov AL,09Bh ; Load the system clock timer out PIT_CNT0,AL ; with a value to generate mov AL,02Eh ; roughly 100 ticks/second out PIT_CNT0,AL ret SetupPIT endp ;----------------------------------------------------------------------------- ; Main - Main program start point. ;----------------------------------------------------------------------------- Main proc far call DiskOff ; Turn floppy drive off mov AX,BootData ; Load the data segment mov DS,AX mov AX,seg initStack ; Load the kernel data mov ES,AX ; segment in order to lss SP,ES:initStack ; load the new stack mov EAX,0 ; Convert the data segment mov AX,BootData ; into a 32-bit linear shl EAX,4 ; address and store for mov segBase,EAX ; use by the LINEAR macro pushfd ; Make sure no surprises are pop EAX ; found in the flags, clear and EAX,00FFFh ; VM, IOPL, NT, and so on push EAX popfd cli ; Ensure interrupts disabled call SetupGDT ; Setup the GDT table call SetupIDT ; Setup the IDT table call SetupSP ; Setup the SP, A20 hardware call SetupPIC ; Setup the PIC hardware call SetupPIT ; Setup the PIT hardware lgdt fword ptr GDTInfo ; Load the GDT lidt fword ptr IDTInfo ; Load the IDT mov EAX,CR0 ; Switch to Protected Mode or AL,1 ; by setting the PM bit mov CR0,EAX db 0EAh ; Perform a far jump to clear dw offset KernelStart ; the prefetch queue and dw selKCS ; load the kernel segment Main endp Boot ends end main