.

1 files changed, 10 insertions, 12 deletions

diff --git a/boot.subx b/boot.subx
index c937e700..4258b206 100644
--- a/boot.subx
+++ b/boot.subx
@@ -916,17 +916,15 @@ Font:
 # offset 1800 (address 0x9400)
 == code 0x9400
 
-# Use 28-bit PIO mode to transfer a string spanning at most one sector (512
-# bytes) of data.
+# Use 28-bit PIO mode to read the first sector (512 bytes) from an IDE (ATA)
+# disk drive.
 # Inspired by https://colorforth.github.io/ide.html
 #
 # Resources:
 #   https://wiki.osdev.org/ATA_PIO_Mode
 #   https://forum.osdev.org/viewtopic.php?f=1&p=167798
 #   read-sector, according to https://www.scs.stanford.edu/11wi-cs140/pintos/specs/ata-3-std.pdf
-#
-# Currently assumes top 4 bits of the 28-bit LBA coordinate are 0.
-load-sector-string-from-primary-bus-secondary-drive:  # LBAlo: byte, LBAmid: byte, LBAhi: byte, out: (addr stream byte)
+load-first-sector-from-primary-bus-secondary-drive:  # out: (addr stream byte)
   # . prologue
   55/push-ebp
   89/<- %ebp 4/r32/esp
@@ -937,12 +935,12 @@ load-sector-string-from-primary-bus-secondary-drive:  # LBAlo: byte, LBAmid: byt
   # check for drive
   (secondary-drive-exists?)  # => eax
   3d/compare-eax-and 0/imm32/false
-  0f 84/jump-if-= $load-sector-string-from-primary-bus-secondary-drive:end/disp32
+  0f 84/jump-if-= $load-first-sector-from-primary-bus-secondary-drive:end/disp32
   # kick off read
   (ata-drive-select 0xf0)  # primary bus, secondary drive; 4 LSBs contain 4 upper bits of LBA (here 0)
   (clear-ata-error)
   (ata-sector-count 1)
-  (ata-lba *(ebp+8) *(ebp+0xc) *(ebp+0x10))  # lower 24 bits of LBA
+  (ata-lba 0 0 0)  # lower 24 bits of LBA, all 0
   (ata-command 0x20)  # read sectors with retries
   # poll for results
   (poll-ata-primary-bus-primary-drive-regular-status-word)
@@ -954,20 +952,20 @@ load-sector-string-from-primary-bus-secondary-drive:  # LBAlo: byte, LBAmid: byt
     74/jump-if-= break/disp8
     ed/read-port-dx-into-eax
     # write 4 bytes to stream one at a time
-    (append-byte *(ebp+0x14) %eax)
+    (append-byte *(ebp+8) %eax)
     49/decrement-ecx
     c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
-    (append-byte *(ebp+0x14) %eax)
+    (append-byte *(ebp+8) %eax)
     49/decrement-ecx
     c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
-    (append-byte *(ebp+0x14) %eax)
+    (append-byte *(ebp+8) %eax)
     49/decrement-ecx
     c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
-    (append-byte *(ebp+0x14) %eax)
+    (append-byte *(ebp+8) %eax)
     49/decrement-ecx
     eb/jump loop/disp8
   }
-$load-sector-string-from-primary-bus-secondary-drive:end:
+$load-first-sector-from-primary-bus-secondary-drive:end:
   # . restore registers
   5a/pop-to-edx
   59/pop-to-ecx