From 6cfbee2e01a9efa620ab43321813bcedbc100747 Mon Sep 17 00:00:00 2001
From: Steve Kemp <steve@steve.org.uk>
Date: Tue, 16 Apr 2024 06:05:06 +0300
Subject: [PATCH] Implement sequential file read

This pull-request implements the sequential block-read operation,
and closes #22.

With this I can restore thes aved-state I created in #23:

```
>restore
Load SAVE disk then enter file name.
(default file name is ZORK1.SAV).
Type <ENTER> to continue  > FOO

Load Game Disk if it was removed.
Type <ENTER> to continue >

Ok.

>inventory
You are carrying:
  A glass bottle
  The glass bottle contains:
    A quantity of water
  A leaflet

>look
Kitchen
You are in the kitchen of the white house. A table seems to
have been used recently for the preparation of food. A passage
leads to the west and a dark staircase can be seen leading
upward. A dark chimney leads down and to the east is a small
window which is open.
On the table is an elongated brown sack, smelling of hot
peppers.

>quit
Your score is 10 (total of 350 points), in 20 moves.
This gives you the rank of Beginner.
Do you wish to leave the game? (Y is affirmative): >y
```
---
 cpm/cpm.go          |  4 +++
 cpm/cpm_syscalls.go | 73 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 76 insertions(+), 1 deletion(-)

diff --git a/cpm/cpm.go b/cpm/cpm.go
index 9e68279..8bc6bc2 100644
--- a/cpm/cpm.go
+++ b/cpm/cpm.go
@@ -149,6 +149,10 @@ func New(filename string, logger *slog.Logger) *CPM {
 		Desc:    "F_DELETE",
 		Handler: SysCallDeleteFile,
 	}
+	sys[20] = CPMHandler{
+		Desc:    "F_READ",
+		Handler: SysCallRead,
+	}
 	sys[21] = CPMHandler{
 		Desc:    "F_WRITE",
 		Handler: SysCallWrite,
diff --git a/cpm/cpm_syscalls.go b/cpm/cpm_syscalls.go
index 5554082..d7089dd 100644
--- a/cpm/cpm_syscalls.go
+++ b/cpm/cpm_syscalls.go
@@ -354,6 +354,77 @@ func SysCallDeleteFile(cpm *CPM) error {
 	return err
 }
 
+// SysCallRead writes a record to the file named in the FCB given in DE
+func SysCallRead(cpm *CPM) error {
+
+	// Don't have a file open?  That's a bug
+	if !cpm.fileIsOpen {
+		cpm.Logger.Error("attempting to write to a file that isn't open")
+		cpm.CPU.States.AF.Hi = 0xff
+		return nil
+	}
+
+	// The pointer to the FCB
+	ptr := cpm.CPU.States.DE.U16()
+	// Get the bytes which make up the FCB entry.
+	xxx := cpm.Memory.GetRange(ptr, 36)
+
+	// Create a structure with the contents
+	fcbPtr := fcb.FromBytes(xxx)
+
+	// offset
+	BlkS2 := 4096
+	BlkEx := 128
+	offset := int(int(fcbPtr.S2)&MaxS2)*BlkS2*blkSize +
+		int(fcbPtr.Ex)*BlkEx*blkSize +
+		int(fcbPtr.Cr)*blkSize
+
+	_, err := cpm.file.Seek(int64(offset), io.SeekStart)
+	if err != nil {
+		return fmt.Errorf("cannot seek to position %d: %s", offset, err)
+	}
+
+	// Temporary area to read into
+	data := make([]byte, blkSize)
+
+	// Fill the area with data
+	for i := range data {
+		data[i] = 0x1a
+	}
+
+	// Read from the file, now we're in the right place
+	_, err = cpm.file.Read(data)
+	if err != nil {
+		return fmt.Errorf("error writing to file %s", err)
+	}
+
+	// Copy the data to the DMA area
+	cpm.Memory.PutRange(dma, data[:]...)
+
+	MaxCR := 128
+	MaxEX := 31
+
+	fcbPtr.S2 &= 0x7F // reset unmodified flag
+	fcbPtr.Cr++
+	if int(fcbPtr.Cr) > MaxCR {
+		fcbPtr.Cr = 1
+		fcbPtr.Ex++
+	}
+	if int(fcbPtr.Ex) > MaxEX {
+		fcbPtr.Ex = 0
+		fcbPtr.S2++
+	}
+	fcbPtr.RC++
+
+	// Update the FCB in memory
+	cpm.Memory.PutRange(ptr, fcbPtr.AsBytes()...)
+
+	// All done
+	cpm.CPU.States.AF.Hi = 0x00
+	return nil
+
+}
+
 // SysCallWrite writes a record to the file named in the FCB given in DE
 func SysCallWrite(cpm *CPM) error {
 
@@ -506,7 +577,7 @@ func SysCallReadRand(cpm *CPM) error {
 			return 1
 		}
 
-		cpm.Memory.PutRange(0x80, data[:]...)
+		cpm.Memory.PutRange(dma, data[:]...)
 		return 0
 	}