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we _can_ access different locations in the stack. correct this comment, and add an example for clarity

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This commit is contained in:
Grant Gordon 2018-09-21 13:24:55 -05:00
parent 5b88226edf
commit 89bc72d54c
1 changed files with 10 additions and 6 deletions
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16
04-bootsector-stack/boot_sect_stack.asm
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@ -1,19 +1,23 @@
mov ah, 0x0e ; tty mode mov ah, 0x0e ; tty mode
mov bp, 0x8000 ; this is an address far away from 0x7c00 so that we don't get overwritten mov bp, 0x8000 ; this is an address far away from 0x7c00 so that we don't get overwritten
mov sp, bp ; if the stack is empty then sp points to bp mov sp, bp ; if the stack is empty then sp points to bp
; if we push (0x8000 - 0x7c00) bits (0x400 or 1,024 bits), we will start to write over
; the bootloader. so, don't do that!
; still in 16-bit mode, so these each push 16 bits onto the stack.
push 'A' push 'A'
push 'B' push 'B'
push 'C' push 'C'
; to show how the stack grows downwards ; to show how the stack grows downwards
mov al, [0x7ffe] ; 0x8000 - 2 mov al, [0x7ffe] ; 0x8000 - 16 bits
int 0x10 int 0x10
; however, don't try to access [0x8000] now, because it won't work mov al, [0x8000] ; this is a null byte - it's the top of the stack
; you can only access the stack top so, at this point, only 0x7ffe (look above) int 0x10
mov al, [0x8000]
mov al, [0x7ffa] ; 0x8000 - 48 bits. lower in memory
int 0x10 int 0x10