Add mechanism to declare variables as "low mem" and use for extra stack.

Add a mechanism (VARLOW declaration) to make a variable reside in the
low memory (e-segment) area.  This is useful for runtime variables
that need to be accessed from 16bit code and need to be modifiable
during runtime.

Move the 16bit "extra stack" from the EBDA to the low memory area
using this declaration mechanism.  Also increase the size of this
stack from 512 bytes to 2048 bytes.

This also reworks tools/layoutrom.py a bit.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 3 insertions, 4 deletions

diff --git a/src/ata.c b/src/ata.c
index c37691a..b261bfe 100644
--- a/src/ata.c
+++ b/src/ata.c
@@ -10,7 +10,7 @@
 #include "util.h" // dprintf
 #include "cmos.h" // inb_cmos
 #include "pic.h" // enable_hwirq
-#include "biosvar.h" // GET_EBDA
+#include "biosvar.h" // GET_GLOBAL
 #include "pci.h" // foreachpci
 #include "pci_ids.h" // PCI_CLASS_STORAGE_OTHER
 #include "pci_regs.h" // PCI_INTERRUPT_LINE
@@ -379,6 +379,7 @@ struct sff_dma_prd {
 static int
 ata_try_dma(struct disk_op_s *op, int iswrite, int blocksize)
 {
+    ASSERT16();
     if (! CONFIG_ATA_DMA)
         return -1;
     u32 dest = (u32)op->buf_fl;
@@ -396,9 +397,7 @@ ata_try_dma(struct disk_op_s *op, int iswrite, int blocksize)
         return -1;
 
     // Build PRD dma structure.
-    struct sff_dma_prd *dma = MAKE_FLATPTR(
-        get_ebda_seg()
-        , (void*)offsetof(struct extended_bios_data_area_s, extra_stack));
+    struct sff_dma_prd *dma = MAKE_FLATPTR(SEG_LOW, ExtraStack);
     struct sff_dma_prd *origdma = dma;
     while (bytes) {
         if (dma >= &origdma[16])