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Add .seek asm pseudo-op, advances location counter to fixed offset

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The new .seek assembler pseudo-op advances the location
counter to a fixed offset within a section --- or to a fixed
address, if the section is a .base'd section.  It works
somewhat like the GNU assembler's .org pseudo-op, though
with a hopefully less confusing name.

This pseudo-op lets us avoid having to manually compute the
needed boot sector padding in the pc86 start-up code
plat/pc86/boot.s .
This commit is contained in:
Tee-Kiah Chia 2021-03-19 13:36:24 +00:00
parent ced1e91800
commit 64a74b4e09
5 changed files with 16 additions and 14 deletions
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1
mach/proto/as/comm0.h
View file

@ -226,6 +226,7 @@ typedef struct sect_t sect_t;
/* s_flag bits: */
#define BASED 1 /* at fixed position */
#define SOUGHT 2 /* did a .seek in the section */
/* sflag bits: */
#define SYM_EXT 001 /* external symbols */

11
mach/proto/as/comm2.y
View file

@ -59,6 +59,7 @@ static item_t *last_it, *o_it;
%token ALIGN
%token ASSERT
%token SPACE
%token SEEK
%token <y_word> LINE
%token FILe
%token <y_word> LIST
@ -252,6 +253,16 @@ operation
DOTVAL += $2;
DOTSCT->s_zero += $2;
}
| SEEK absexp
{ if (DOTSCT == NULL)
nosect();
if ($2 < DOTVAL)
serror("cannot move location counter backwards");
if (pass == PASS_1)
DOTSCT->s_flag |= SOUGHT;
DOTSCT->s_zero += $2 - DOTVAL;
DOTVAL = $2;
}
| DATA datalist
| DATA8 data8list
| DATAF dataflist

1
mach/proto/as/comm3.c
View file

@ -37,6 +37,7 @@ item_t keytab[] = {
{0, ALIGN, 0, ".align"},
{0, ASSERT, 0, ".assert"},
{0, SPACE, 0, ".space"},
{0, SEEK, 0, ".seek"},
{0, COMMON, 0, ".comm"},
{0, SECTION, 0, ".sect"},
{0, BASE, 0, ".base"},

2
mach/proto/as/comm6.c
View file

@ -142,6 +142,8 @@ newbase(valu_t base)
nosect();
if (sp->s_flag & BASED)
serror("already based");
if (sp->s_flag & SOUGHT)
serror("cannot rebase section after a seek");
sp->s_base = base;
sp->s_flag |= BASED;
DOTVAL += base;

15
plat/pc86/boot.s
View file

@ -12,19 +12,6 @@
.sect .text
! ****** WARNING! ******
!
! The PC boot sector requires a magic number at the end to signify that the
! disk is bootable. Unfortunately, the ACK assembler is a bit simple and we
! can't tell it to put the 0xAA55 at a particular address without writing our
! own custom binary generator. As a result, we need to manually insert just
! the right amount of padding in order to make this work.
!
! If you ever need to change the boot code, this needs adjusting. I recommend
! a hex editor.
PADDING = 0xB3
! Some definitions.
BOOT_SEGMENT = 0x07C0 ! Where we've been loaded
@ -317,7 +304,7 @@ exename: .asciz 'pc86.img'
! ...and we need this to fool the PC into booting our boot sector.
.space PADDING
.seek 0x1FE
.data2 0xAA55
! Define symbols at the beginning of our various segments, so that we can find