From 6ef31cb03460d7b2d9aa9640eaa9e6a076431a2e Mon Sep 17 00:00:00 2001
From: Neil Webber <neil@webber.com>
Date: Wed, 10 Apr 2024 07:11:03 -0500
Subject: [PATCH] Checking in an intermediate LDA implementation but going to
 change it to conform 100% w SIMH

---
 boot.py | 113 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 113 insertions(+)

diff --git a/boot.py b/boot.py
index 9b2a281..6248c1d 100644
--- a/boot.py
+++ b/boot.py
@@ -117,6 +117,119 @@ def boot_bin(p, fname, /, *, addr=0, deposit_only=False,
     return addr if deposit_only else None
 
 
+def _must_read_n(f, n):
+    """read exactly n bytes from f; raise exception if can't. n==0 allowed."""
+
+    if n == 0:         # read(0) works but feels dirty
+        return bytes()
+    b = f.read(n)
+    if len(b) != n:
+        raise ValueError(f"needed {n} bytes; got {len(b)}")
+    return b
+
+
+def _byte_phys_write(p, b, addr):
+    """write b (bytes()) to addr. b may be odd length. addr may be odd."""
+
+    # addr can be odd, length can be odd. Pick up the bookend bytes
+    # as needed so physRW_N can do it as words.
+    if addr & 1:
+        addr -= 1
+        b = bytes([p.physRW(addr) & 255]) + b
+    if len(b) & 1:
+        b += bytes([(p.physRW(addr+len(b)-1) >> 8) & 255])
+        
+    words = [(b[i+1] << 8) | b[i] for i in range(0, len(b), 2)]
+    p.physRW_N(addr, len(words), words)
+
+
+def load_lda_f(p, f):
+    """Read open file f as an 'absolute loader' (.LDA, sometimes .BIC) file
+    This is the same format that simh defines for its load (binary) command.
+
+    Returns: the address specified in the END block (per LDA docs)
+
+    Any file format errors or I/O errors will raise an exception.
+    """
+
+    # An LDA/BIC/absolute loader file may start with an arbitrary number
+    # of zero bytes (possible w/paper tape). Any leading zero bytes will be
+    # ignored. After that it should be repeated blocks:
+    #   [HEADER]          -- 6 bytes
+    #   [DATA]            -- size determined by header, can be zero length
+    #   [CHECKSUM]        -- 1 byte
+    #
+    # The header is six individual bytes, as follows:
+    #         1             -- literally, a byte with value 1
+    #         0             -- "MUST" be zero (this code ignores this byte)
+    #       len-lsb         -- lower 8 bits of block length
+    #       len-msb         -- upper 8 bits of block length
+    #       addr-lsb        -- lower 8 bits of address
+    #       addr-msb        -- upper 8 bits of address
+    #
+    # The 'block length' includes the header bytes but not the checksum.
+    # Thus the lengthof [DATA] is six less than the block length given.
+    # Note that the [DATA] length is allowed to be odd, or zero.
+    #
+    # If the [DATA] length is zero (block length 6) the block is
+    # an "END" block that terminates the format. The address indicated
+    # is the "start address" and is returned.
+
+    while (header := _must_read_n(f, 1)) == 0:
+        pass
+
+    header += _must_read_n(f, 5)     # form the rest of the first header
+
+    while True:
+        if header[0] != 1:
+            raise ValueError(f"header starts with {header[0]} not 1")
+
+        count = (header[3] << 8) | header[2]
+        addr = (header[5] << 8) | header[4]
+
+        if count < 6:
+            raise ValueError(f"header error, {count=}")
+
+        b = _must_read_n(f, count-6)
+
+        chksum = _must_read_n(f, 1)
+        if (sum(header) + sum(b) + sum(chksum)) & 0xFF:
+            raise ValueError(f"checksum mismatch, {header=}")
+
+        if count == 6:
+            return addr
+
+        _byte_phys_write(p, b, addr)
+        header = _must_read_n(f, 6)
+
+    assert False, "unreachable"
+
+def boot_lda(p, fname, /, *, force_run=True):
+    """Load and boot an LDA/BIC/absolute-loader file.
+
+    By default, the loaded code is started even if the start address
+    given in the LDA file is odd (which is normally a flag to not start it).
+    The start address is rounded down 1 to even in such cases.
+
+    To override that behavior, specify force_run=False. The file will only
+    be run if the start address is even.
+
+    In all cases the raw start address is returned; however, if the loaded
+    code is successfully started there will never be a return unless that
+    code eventually halts.
+    """
+    with open(fname, 'rb') as f:
+        addr = rawaddr = load_lda_f(f)
+
+    if rawaddr & 1:
+        if not force_run:
+            return rawaddr
+        addr = rawaddr - 1
+
+    p.run(pc=addr)
+    return rawaddr
+    
+
 def make_unix_machine(*, loglevel='INFO', drivenames=[]):
     p = PDP1170(loglevel=loglevel)